#help-13
1 messages · Page 192 of 1
Here are some other perspectives to perhaps build instinct:
It's similar to the: here are 3 jacks and 1 king. Go around the table letting people pick and keep a card at random and the winner is the one who gets the king.
some people think that picking earlier is better because if you win, then the others don't even get to play
but 1/4
= 3/4 * 1/3
= 3/4 * 2/3 * 1/2
Another way to think about it is this:
pick the first ball but don't look at it. What's the EV of the second ball?
if the X1, X2, X3 represent the number on each ball here, they have identical distributions

Linearity of expectation is so weird
linearity of expectation and good probability intuition are like the same thing
even if there is dependence structure between the random variables, when considering their expectation you can look at each quantity in isolation
The two examples O dog gave are intuitive, but this one feels unintuitive
if you ignore the information about X1 and X3, X2 will behave just as it does when you consider the additional information
the important thing here is the marginal distributions of each random variable
there's no cancellation per se, the distribution of X2 is what it is regardless of what X1 and X3 are
Another way to think about it is to enumarate B1, B2,... ,B9
Then how you pick 3, there will be an equal amounts of cases with each B as X1, and so on
Then EV will take the average
You can assign each B value to be whatever since theure equal counts
expectation is integration and then there's no surprises 
everyone knows integration is linear
@civic coral Has your question been resolved?
ik its brilliant, but this is the first place I read about this and immediately got it
. Didn't really click before
https://brilliant.org/wiki/linearity-of-expectation/
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vicks
I have no idea where to start
well I can try something but tbh I am not sure if it is even right
can anyone guide me
@mint nimbus Has your question been resolved?
nooo 😦
@mint nimbus Has your question been resolved?
nope
To find the probability that the 1st bin is not empty, we subtract this probability from 1
sooo
1 - (1 - 1/n)^k.
no?
p(1st bin is not empty)
that's what i was thinking too
but the whole thing raised to k
not 2nd term
but I'm not sure
you have the answer?
nope
well i would go with those i provided as they are my final answers
can you explain why you only raised the second term by k?
ummm
because we want to find the probability that the 1st bin is not empty after throwing all k balls into n bins
waiyt
Since each ball is thrown independently and uniformly at random into one of the n bins
the probability that any particular ball is not placed in the 1st bin is (n-1)/n.
Now
if you have k balls
and each has this probability of (n-1)/n of not being placed in the 1st bin
then the probability that all k balls
((n-1)/n)^k
okay i see
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Hello, can anybody explain me this? I’m studying probability and the professor made an example and said.. let’s say we want to get all the particles position and velocity in the room, then he proceeded to represent the set of particles as such
That looks like a map of China
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help regarding metrics
Im looking at the metric $$d(x,y)=min{p(x,y), 1}$$
with $p(x,y) = |x-y|$.
for the triangle inequality there should be 4 cases if im not mistaken or not?
triangle equality being: $$d(x,z) <= d(x,y) + d(y,z)$$
$$ 1. p(x,z), p(x,y) and p(y,z) < 1$$
$$ 2. p(x,z) >= 1 and p(x,y) and p(y,z) < 0 $$
$$ 3. p(x,z) >= 1 and p(x,y) or p(y,z) >= 1 $$
$$ 4. p(x,z), p(x,y) and p(y,z) >= 1 $$
Eichhorst
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hey
is someone here
?
I have a string 1 2 3 4 5 6
How many arangements can be done to this string that no number in there can be to its initial position (1 can t be on position 1 2 can t be on position 2 and so on)
this is my problem
I reckon finding the opposite is easier
How many ways would I still have numbers in their original spot
how come
Idk just a hunch
Have you tried anything?
kinda
show it
Let’s see it
ok first i need to translate
!status
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To solve the problem, we will use the principle of bijectivity, also known as the Inclusion-Exclusion Principle.
We have an initial string: 1 2 3 4 5 6.
In order for no element to be in its original position in the resulting strings, we must arrange the elements in such a way that we have no permutations (arrangements) where any element is the same position as in the original string.
We take each element and see how many ways we can put it in its original position. Next, we apply the Inclusion-Exclusion Principle to calculate the total number of possible arrangements that satisfy the required condition.
Item 1: We have 5 positions available to put 1 because it cannot be in its original position. We have 5 ways to arrange element 1.
Element 2: If 1 has taken position 2, then we have 4 positions available to put 2 (apart from its original position). If 1 took position 3, then we also have 4 positions available to put 2 (all apart from its original position). So in total we have 2 * 4 = 8 ways to arrange element 2.
Element 3: If 1 occupies position 3 and 2 occupies position 1, then we have 3 positions available to put 3 (apart from its original position). If 1 occupies position 3 and 2 occupies position 4, we also have 3 positions available to put 3 (besides its original position). If 1 occupies position 4 and 2 occupies position 1, we also have 3 positions available to put 3 (besides its original position). If 1 occupies position 4 and 2 occupies position 3, we also have 3 positions available to put 3 (besides its original position). So in total we have 4 * 3 = 12 ways to arrange element 3.
Element 4: Similarly, we will get 20 ways to arrange element 4.
Element 5: Similarly, we will get 30 ways to arrange element 5.
Element 6: Similarly, we get 30 ways to arrange element 6.
Finally, we add up the total number of ways we can arrange each element separately: 5 + 8 + 12 + 20 + 30 + 30 = 105.
Therefore, in 105 different ways, we can arrange the elements of the original string so that none of them is in its original position
dude we wanted to see what you have tried
Was what you tried “ask chatgpt”
Because if it is, that’s not training your own thinking skills
And if that’s not what you tried
i got like 6 diff answers to that
Neb1, never look up the solutions or ask gpt for combinatorics and calculus.
Or basic arithmetic
@twilit birch if 1 is not allowed at the 1st position, how many possibilities does the 1st position have?
5! ?
why 5!
oh just the first pos
we have 5 more digits so 5 more posibilities no?
yeah
now the same on 2nd position
how many possibilities do we got if 2 isn't allowed?
5
isn't this the same for all 6 positions?
no because i can t have idk like 1 3 2 4 5 6
because there ARE STILL 4 digits that are in their same exact pos
1 4 5 6 are in their initial position
alr
just rearranging them
i found something about idk fixed point arrangements
or smth like this
i might think it s related
these are the answers that are given
so?
back, had to go for a while
no problem man
i translated, the question does not say anything about repetition
look i m not that good at these things with combination and all that but it s reffering to an array
it s a problem at a supposed comp science exam
and it s not the maths exa
exam
I have not done combinatorics since a year but I'll try
ok
exam?
kinda weird since I'm getting 5×5!
not sure where I'm making the mistake
at least you tried man
i know right?
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,tex (sqrt(3) x sqrt(50)) / sqrt(6)
Iver
How do I simplify?
I already made sqrt(50) into 5sqrt(2)
Look at brackets or whatever they are called
Carefully, because I divded everything by sqrt(6)
sqrt(6) = sqrt(2) * sqrt(3) since sqrt(ab) = sqrt(a) * sqrt(b)
Use that to your advantage
omg how i didint think of that
thank you
let me see if i can figure out now
,tex 5
Iver
@cosmic steppe ?
,w 150/6
Yeah
what is this?
Just checking
3 * 50
your fraction is equivalent to sqrt(150/6)
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Hi.
Just small question about binary system
So their is 4 type , binary and octal (8) and decimal (10) and hexadecimal (16)
So my question is about this (5) and (4) for example what they call it , and also I can do calculate In it using only fhe devide method on it? (I mean for example for types above their is others methods to convert)
So their is only devision method for base like This?
i cant say
i know only that method too
although im comfortable with powers and use those
Oh ok
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why are the two steps marked wrong, but the answer is correct
because w = x^2 - 36, not w = x^2
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Conservation of mass related question I guess,
I'm trying to find the concentration of a drug within 5L of blood, where it is being filtered out at a rate proportional to its concentration.
The initial dose is 200mg at 4mg/s, and is followed by a continuous administration at a rate of 0.25mg/s, which balances the rate at which it is filtered out.
It's assumed that the level of blood is constant, and the concentration stays constant during the 0.25mg/s phase, im just wondering how to find that concentration.
Sorry if there is not enough information, this is just driving me up a wall
This is for an intro mathematical modelling course if that helps at all
@silver osprey Has your question been resolved?
<@&286206848099549185> when you get a chance 
ooh thats actually an intresting question
its for homework and alot of my classmates are going with 40mg/L but my thought process is that it would start filtering the second its administered so it would be abit less once the full 200mg dose is done
concentration is measured in mg/L?
ye
a rough diagram that to visualise my thought process i did but that’s about as far as i’ve gone
could never really wrap my head around conservation of mass equations in modelling
i got 39.95 😭
wtf
could you show me how you got to that? im confused as to how to approach it
ok so 200 mg at 4 mg/s is administered, which would take 50 seconds, right?
lets model everything as a function of time, so C(t) = concentration at a time
f(t) = rate of filtering as a function of time
f(t) = k*C(t) (via proportionality)
now lets work out C(t)
C(t) = (mg of drug)/5
to find the mg of the drug at any point, we know were getting +4 mg every second, but losing f(t) mg every second
so then mg of drug = 4t - f(t)
so C(t) = (4t - f(t))/5
and f(t) = k*(4t - f(t))/5
rearrange it to get f(t) = 4t/(5/k + 1)
so now we have f(t) in k and 1
ooh ok
ok i overcomplicated this alot
once you get C(t) = (4t - f(t) )/5
thats all u need to find
ignore all the proportionality stuff
we know f(50) = 0.25
so when t = 50, f(t) = 0.25 (cuz adding 0.25 balances it)
sub that here
you get C(50) = 39.95
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✅
actually sorry lmao
how would i find K moving forward?
also since f(t) is a rate does subbing it into C(t) = (4t - f(t) )/5 as 0.25 work?
sorry just trying to wrap my head around it
lmao nice bot
<@&268886789983436800> 
yes you can sub f(t) = 0.25 and t = 50 (once all 200 mg have been used)
this is how i found C(t) at t= 50
but that wont help you find k
mm ok
for k you need to consider the proportionality
you have a function for C(t) in terms of f(t) and t
but is 0.25 not whats being removed at t=50 not what has been removed in total?
write f(t) = k*C(t) , so then you have an equation in f(t), t and k. then sub known values of t and f(t) (like 50 and 0.25)
after the 200 mg is injected after 50 seconds, 0.25 mg/sec is administered. so its not a total, but a rate per second. since it perfectly counteracts the rate of filtration, we can assume f(t) for that period = 0.25, so whatever it being removed is being perfectly accounted for to keep concentration constant
oof
you should get some sleep
come back to this in the morning
ping me if you need any more help
yeah, ive got the class for it at 1pm so i was just trying to crunch it out before then, its been a hectic weekend
thankyou again for your help
you're amazing
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any time
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Does this look correct so far?
Usually one would just substitute 3sin u = x
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how do i solve this?
<@&286206848099549185>
so what do i do
they gave you the surface area of it
yeah
so they're equal
i need to find the diameter
first you find the radius, just multiply it by 2 to get the diameter
you have the "build" an equation
how
you have the formula for surface area of a hemisphere, and you have the value of it
so you just make them equal
356= 2pi r^2 + pi r^2
ok so we divide it by 2pi?
yep
accident
and find the square root
then multiply the radius by 2 to get the value of the diameter
yep
so when i put in in my cacl
but you want to find the diameter, not the radius
diameter = 2 * radius
exactly
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Can someone help idk if I’m doing it right
8-1+4*2 all over 10
So 15/10
Or 1 and a half
Just sudstitute what you found G(2) and G(-1) to be
so part 1 and 2 are right
Yup
and for part 3 i do this
Yes
ok hold up
HadarS
$\frac{8-(1)+8}{10}$
HadarS
$\frac{15}{10}$
HadarS
3/2
$1\frac{1}{2}$
HadarS
Yes
I love this Tex bot
ty for the help
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What does it mean for a sequence to be increasing and bounded
increasing means the terms are getting bigger
bounded means there's a number that's greater than all terms in the sequence
a_n is the sequence yes
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Can someone help me figure out where I messed up?
It said the answer is supposed to be -5ln|x+2| + 5ln|x+1| + 4/(x+1) + C
So I am close, I just don’t know where the 5ln|x+1| comes from, and I don’t know how they get rid of the x
fyi this channel's gonna close
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I'm on number 2, and this is what I have so far
I don't think the set spans R^2...
but I'm not sure
I haven't learned how to figure it out with like linear dependence or something yet
I can say (1, 0) where 1=1+2(0)+2(0)
and a=1, b= 0, c=0
so then
y=-1 -2(0) + 3(0)
that doesn't equal 0
so (1, 0) doesn't work there
but I dont know if it can work with another combination of a, b, c
so idk if that's a valid counterexample to say the set doesn't span R^2
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@hollow ember Has your question been resolved?
the sets elements arent all linearly independent
(2,-2) is a multiple of (1,-1)
therefore they can be written as linear combinations of eachother
So does that mean the set doesn’t span R^2?
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I would've done volume - surface area but the surface area is bigger so
I don't know what to do
The answer is A
a 5 by 5 cube is painted black on the surface
means the inner 3 by 3 is not painted in black
You simply just substract the side by 2 to get the side of the inner cube
Oh yeaaaaa
That makes sense
Okay ty
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does anyone know the equation of a skewed quadratic? or a similar function
wdym
@edgy laurel Has your question been resolved?
i’m trying to find a function that can model increasing velocity and decreasing velocity, but it can’t be a quadratic. it needs to increase quickly, reach a max and then decrease at a slower rate
Something like 1 - x^4 perhaps?
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y is inversely proportional to the square root of x.
It is known that y = 64 for a particular value of x.
Find the value of y when this value of x is increased by 300%.
i only know the formula is y=k(sqroot x)
no
inversely proportional
it's $y = \frac{k}{\sqrt{x}}$
Ann
do you know what "increased by 300%" means?
multiplied by 4?
is it divided by 4
Ann
yes
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$$Prove that \sum_{k=0}^{n}(-1)^k $$
$$\begin{smallmatrix}
n \
k$$
Please don't occupy multiple help channels.
$$Prove that \Sigma_{k=0}^{n}(-1)^k \binom n k= 0$$
$\binom n k$
thanks
Denascite
and \sum instead of \Sigma
Prove that $\sum_{k=0}^n (-1)^k \binom{n}{k} = 0$.
Ann
here, fixed the badtex for you.
and dont put the text inside the $
!status
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1. I don't know where to begin
2. I have begun but got stuck midway
3. I got an answer but I'm told it's wrong
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6. None of the above
thankyouu
do you know the binomial theorem?
ye
I'm just absolutely terrible with proving stuff and have no idea how to even begin with such things
ok can you state the binomial theorem for me
$\binom n k = \frac{n!}{k!(n-k)!}$
Feevo
oh whooops
Feevo
there we go
now, what values can you pick for x and y to make the RHS of this into $\sum_{k=0}^n \binom{n}{k} (-1)^k$?
Ann
so I basically have to find $x^{n-k}y^k=(-1)^k$
Feevo
cringe wording but sure...
x and y can be whatever you want
what could you make them into so that x^(n-k) y^k becomes (-1)^k?
for the love of god do not see this as equation-solving.
if x is 0 and y is -1 ig
You’re telling me (0 - 1)ⁿ = 0?
not me forgetting that this is multiplication
1 and -2, for instance then
no
idfk
x=1, y=-1
i am dumb
You just aren’t as experienced
or both 😛 (but it's too early to tell if u r dumb
)
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i'm confused with 7b
i think what i did is wrong because i'm double counting stuff and also every time you add a man, it creates more gaps
so are there 16x17x18x19 ways to organize the remaining 4? instead of 16^4
and i don't know how to solve the double counting problem :( am i apporaching it wrong?
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Corroll me plese
,rccw
is correct?
2,8 a)true b) true c) false d) true e) true f)false
empty set is a subset of {0}
not a element
So 2.7 b is ?
What does $\emptyset \in {0}$ mean ?
rafilou2003
rafilou2003
Is there a difference with $\subseteq$?
rafilou2003
okay, there is a difference
give me a sec
in short terms,\subsetteg is like =
so , if we have 2 sets like A = {1,2} and B = {1} B is subset of A but not ⊆
if B was {1,2} then i can use ⊆ symbol
correct?
Wait what the empty set is in every set
empty set is subset of every set
2.7 B is an example . So, that expression false?
${0}\cap{1} = \emptyset$
i don't know how to read that
Do you agree
Frosst
no
Then what should the intersection be
?????
No. The set containing the empty set, for example, is not a subset of the empty set
The empty set itself is a subset of every set
Is the empty set not in the empty set
Oh
"If A is a set, every element of the empty set is an element of A". This statement is true because the empty set has no elements
So for any set A, $\emptyset \subseteq A$
rafilou2003
Ah if I select nothing from A I get the subset the empty set
And unions and intersects give sub/supersets not elements
With what was said about the difference between $\subset$ and $\subseteq$, it would be nice to check 2.7 f) and g) again
rafilou2003
And check 2.7 e) as well
What is okay?
my proof
That A is a subset of A ?
A ⊆ C
There is no proof here
You just stated "If A ⊆ B and B ⊆ C, then A ⊆ C"
Or are you talking about what's under
Are you trying to prove that A ⊆ A ???
I am trying to proof A ⊆ C
You're trying to prove {1,2} ⊆ {1,2}
yeah , i just use an example of sets
Ok, what's the definition of "A ⊆ C"?
by exercise is A ⊆ B, B ⊆ C, and I need to proof that A ⊆ C
Ok
.
A is subset or equal to C
And what does that mean?
dont understand a question
What does "A is a subset of C" mean?
It mean , that they have same elements
if B is subset or equal to C and A is subset or equal to C then A is equal or subset of C
i don't how to write it in normall way
What is the definition of "A is a subset of B" ? How do you know, if I give you two sets A and B, that one is a subset of the other?
because it's written in exercise
????
Ok well here's something for you :
If A = {1,2,3} and B = {1,2}, is one a subset of the other? And why?
B is subset of A because A have numbers{1,2} and B have numbers{1,2} so by deffenition of subset subset is a part of a given set
What does it mean to be “a part of a given set”
it means that they have similiar elements
set B has set A elements in it
What does “has” mean
contain
We’re trying to define contain here
How do you know {1,2} is a subset A
And why did you check {1, 2}
i just create 2 random sets
Why didn’t you choose {ball, apples}
what?
Why randomly choose sets
i really don't understand what you want to hear
subset or equal
What does it mean to be equal
Yes, A = B means that "Elements of A are elements of B AND elements of B are elements of A"
$\forall x\in A(x\in A \iff x\in B)$ means $A = B$
Frosst
So now, from this definition of A = B, can you guess how A ⊆ B is defined ?
$\forall x(x\in A \iff x\in B)$ means $A = B$
rafilou2003
$\forall x(x\in A \iff x\in B)$ means $A ⊆ B$
Ah that’s enough as well
Sali_12
No, that would be equals
We just said that this is equals
"Means" is not "implies"
okay, but u need to proof that A ⊆ C
Mhmm I suppose it’d mean is defined as
Yes, but how can we prove it when we dont know what it means?
$\forall x(x\in A \Longrightarrow x\in B)$ means $A ⊆ B$
rafilou2003
Every element of A IS an element of B. Only this, and not the reciprocal, means that A is a subset of B
okay
So now
then i can write $\forall x(x\in A \Longrightarrow x\in C)$ means $A ⊆ C$
Sali_12
Yes
So proving A ⊆ C means proving the statement to the left
How do we prove a statement that starts with "forall" ?
okay, but how to write answear if i have a test
first i need to write
$\forall x(x\in A \Longrightarrow x\in B)$ means $A ⊆ B$
Sali_12
Sali_12
We want to prove this statement
The proof of a statement that starts with "forall" will start with "Let..."
It will ALWAYS be the case
So
"Let x be an element."
Then we have to prove that $x\in A \Longrightarrow x\in C$
rafilou2003
So what do we write first?
proof that A ⊆ B
No, we know that A ⊆ B, that's in our hypotheses
So we don’t have to prove it
We have to prove that "Statement 1" implies "Statement 2"
How do we write the proof of that?
let x E A .
since A ⊆ B , then x E B
since B ⊆ C and x E B , then x E C
since x we take randomly from A , we show that x E C , we proved that A ⊆ C
is that okay?
yes
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Ive set c = omega * e^(alpha t)
got the dervative of that and set it equal to DlapaceC + alpha c
idk what else to do from here
i also have tried taking the dergative of domega/dt
oh wait NEVERMIND
by doing it that way i see that dthea/dt = dlapacanC e^-alphat
i can plug that back in and i get the solution i think 🙂
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How I make this
What do you need help with?
how can I calculate with a?
The first example in a?
wait
Waiting
always ready
I don't understand textbooks that give a subtask a that has 3 examples in it
Forgot a closing bracket there
Oh ok
this is gonna be (2 sqrt(5) - sqrt(3)) * (2 sqrt(5) - sqrt(3))
when you square brackets like this you foil out
Yeah, otherwise only the sqrt(3) is getting squared
Have you learned about binomial expansion formulas yet?
Yeah
USS-Enterprise
How do we expand this
ahh
USS-Enterprise
I have to be real with you, this example is none of the 6 you posted above
LMAO
It however is correct, yes.
I have this in extra page
thats the way you solve these problems though
Sorry
No problem 🙃
you expand it out, do the multiplican and simplfly
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riemann
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I have to prove using the axioms that x < y <=> -x > -y
How to do it? I have no ideas 😭
well use the axioms
copy them here from your book if you don't know them by heart
but i don’t really know which ones i should use
try
that’s the only one assignment that i don’t have solution to
you shouldn't need a solution to do this if you're taking a class that requires you to prove staetments, just axioms
how would you do this if you didnt have to use axioms explicitly
i have to use them in this example
...
i have four examples to do and 3/4 are done
dena is giving you a helpful hint
that’s the last one
and i asked you to post your axioms
and yet you've done neither
what do you want from helpers
i would multiply them by -1
ok. now post your axioms
try using the axioms that have multiplication and -x or -y
x + (-x) = 0 is the only one with -x
I would instead try to do it only with addition
so, you need at least one axiom about < which you should use. if I say "only addition", which one will it be
12
aka "add on both sides". what could be useful to add here
the opposite of one of the factors
0 < y - x and 0 > x - y
only do it to the left inequality
the one we start with
we want to do something to it
and end with the other one
ok, so 0 < y-x
where we used axiom 4
now, what else could we add
yup
0
well adding zero does not that much
youre welcome
Can someone help me to solve an integral ?
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For two n-vectors a,b, is a x transpose(b) necessarily invertible? trying to show that C = B + a x transpose(b) is invertable, given B is invertable
all the rows of a*b^T are multiples of b^T
so if you do row reduction, you'll just get a single nonzero row
your claim is not true in general. you need to know something about a and b in relation to B
well at least thats exactly the correct condition
try solving (B+ab^T)x=y for x
let s = b^Tx, solve it in terms of s
x = B^-1 (y-as)
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how do i get started
@cyan void Has your question been resolved?
above
You just need the value of the angle DXG? Because with the value of DXF, the value of FZG; and G is the incenter of the triangle XYZ, I understand that it would get its value of the angle DXG.
hm
Would it be 36
The value of DXG it would be 46°
still confused
Wait
Do you understand that now?
It seems strange to me that it's just like that, because the other data would be filler xd
this is the definition of the incenter, with this and with the value of the DXF angle, it would already be
That makes sense
it not 96
i thought it was centriod
would it be 68
this is another problem?
ok ok
did you figure it out
I'm looking at how to solve it, for now I've gotten DE = 128.

DE = 128; EY = 60; DW = 68
yay
ahh 31 + 59 = 90
yes
I'm not entirely sure, because theory always has its between the lines, but I think they would be 1, 3, 4
ah it was these 2
Yes, LN is not a segment bisector, i was wrong about that.
ahh
Ahh i was wrong about 3 haha
It's confusing to me that it's in English haha but it was still a matter of reading it better
😭
why the emoji?
y = -74; z = 90; x = -34
but you need to study, you may not like it simply because you have not found a pleasant or comfortable way to study
i can do the other parts just not geometry and proofs
and why not geometry and proofs?
i forget all the vocabulary
all dis stuff
A matter of studying it little by little, the same thing happens in algebra, trigonometry, etc.
my brain gets fried after all this
the other algebra stuff is a bit more understandable but this is a different level
i finished basically all my hw besides proofs
well
oh ok ok, the same thing happens to me with geometry
do you know how to do proofs
😭
You forgot say closed
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Unsure what this is asking



