#help-42
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@bold sedge what technique is best to solve this integral
u sub?
nvm
its not u sub
Yes. Substitute 1-y=u
oh..
You can do it using u sub easily
not by doing partial fraction decomp?
idk but with u sub 1+y wills tay the same
Yes
Thats why do this.
i just get -1(1+y)^2 du
-(1+y)^2/u du
You should replace 1+y in terms of u.
idk how to
If 1-y=u then
1-u=y
so 2-u=1+y
okay i get (-2+u)/u
Again wrong
bruh
bathroom mug
bathroom mug
I hope you can do on your own now.
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forgot the measure once :P
lebesgue measure?
Unless you are doing measure theory
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could you send a paper about laplace pressure of uneven shapes?
(my question timed out twice so ill ping <@&286206848099549185> )
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6 persons enter an elevator and pick - independently and with uniform probability - one of 8 floors.
Calculate the expected number of floors on which some number of people (>0) leave the elevator.
Note: Consider a random-variable that is 1 exactly when a person gets off on this floor and 0 otherwise.
First find the probability with which this variable is 0, then its expected value.
With this expected value you can easily calcaulte the result we're looking for.
I'm not sure where to even begin with this one
The random-variable they're looking for for each floor is 0 exactly when all 6 people choose to not get off at one (out of 8) given floors so with probability (7/8)^6 ?
so it's 1 with a probability of 1-(7/8)^6
so since it's binary that's the expected value here?
now how does that help with finding the expected number of floors?
@coarse minnow Has your question been resolved?
We can make another random variable that represents the number of floors people choose to get off the elevator
Xt = X1 + X2 + ... X8
The actual distribution of this random variable is probably very difficult to get. However, the expectation is easy.
@coarse minnow
is the expected value the sum of the individual expected values?
Exactly
so approximately 4.41?
$\sum_{i=1}^8 1-(\frac{7}{8})^6 = 8(1-(\frac{7}{8})^6) = \frac{144495}{32768} \approx 4.41$
Bob Goldham
this?
Yeah it makes sense to me
probabilities making intuitive sense usually means that they're wrong..
It seems easier than it should be, haha. Expectation is easy like that. I have the urge to code this and see the result
I just did that
#!/usr/bin/env python3
import random
LEVELS = [1,2,3,4,5,6,7,8]
def run_once(n_person: int = 6)-> int:
chosen_levels = set()
for i in range(n_person):
chosen_levels.add(random.choice(LEVELS))
return len(chosen_levels)
def mean(data):
n = 0
mean = 0.0
for x in data:
n += 1
mean += (x-mean)/n
if n < 1:
return float('nan')
return mean
def main():
results = []
for i in range(1000000):
results.append(run_once())
print(mean(results))
if __name__ == "__main__":
main()
This does consistently produce values around 4.41, so I'll assume the solution is correct
thank you @civic dirge !
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The different birthdays of 𝑛 people must be at least 3 days apart from each other. Determine approximately
𝑛 such that the probability of this happening is 50%
why 365 - 3(n-1)?
i dont know i just came up with that
yeah it's not correct
why n=2?
for example
so that you see what's wrong
the probability that two birthdays are at least 3 days apart from each other is 365*358/365^2, can you see why?
shouldnt be 365*(365-3)/365^2?
no
we remove 3 days for both of them
hahaha im good at this
which birthdates are LESS than 3 days apart?
this is 365/365 right?
yes
it doesn't matter but it's for representation
which birthdates are NOT at least 3 days apart from june 10th?
7-8-9, 11-12-13
you're missing something
7-8-9, 11-12-13 this is the 3 days that 10 is apart from
ok, so many things wrong with that sentence
so
june 7th, yes it's not at least 3 days apart
because |7-10| = 3, is not greater than 3
same reasoning for 8,9, 11,...
but you missed one
i cant figure it out
don't look far
the days that are not at least 3 days apart from june 10th are not further "before" june 7th and not further "after" june 13th either
but you still missed one
7,8,9,11,12,13
are those the ONLY days that are 3 days apart or less from june 10th?
yes but which day do we remove as well?
10?
YES!
YESSS
finally
10 is not at least 3 days apart from 10
ok
so 7 days
we remove 7 days
so 358
yes
what's the probability when n=1
that all birthdays are 3 days apart or more from each other
365/365?
358/365
365(365-7)/365^2
yes yes
365(365-7)(365-7 - 7)/365^3
there we go
this is ugly though
well it's what it is
(though fair warning : it's not the EXACT formula)
but it's a very very good approximation
can u think something that will make it into a formula
rafilou2003
this formula gets worse and worse at approximating the probability as n gets bigger
but it should hold enough accuracy until we reach 50%
sorry
anyways you get 48% with n = 9
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A country’s gross domestic production is modeled by the equation
P = 2L^3/5K^2/5 where L is the amount the country devotes to labor
and K is the amount spent on capital. Currently the country spends
5 trillion dollars on labor and 4 trillion on capital. If the country is
currently increasing their labor expenditures at a rate of 0.1 trillion dollars/year and their capital expenditures at 0.15 trillion dollars/year, at
what rate is the gross domestic production of the country increasing?
what have you tried?
I guess I otter change how I ask that.
tbh, I missed the online class where my professor taught us this type of problem so idrk where to start
& I'm meant to show each step and explain it
it's asking for a rate, do you know how to find the rate of something?
Is it the derivative?
yup. Find the derivative of P to find the rate
and since K and L are given as functions of time, you should use implicit derivation when you do
So I would take the derivative of the equation, then plug in values for K & L ?
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you can try the rational root theorem
were you doing a division by (x-1)?
hm
| -x^3 -3x^2 0 4
x|-x^4 -3x^3 0 4x
-1 |x^3 3x^2 0 -4
(x-1)(-x^3-3x^2+4)
just seems like a small error
-4 instead of +4
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is there a difference really between these two questions other than the greater than or equal vs greater than?
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for part c of this question, do I just have to figure out $\frac{dz}{dt}=\frac{\partial{z}}{\partial{x}}\frac{dx}{dt}+\frac{\partial{z}}{\partial{y}}\frac{dy}{dt}$
Secret
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Why answer is C?
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<@&286206848099549185>
why shouldn't it be continuous at zero?
Does {.} Mean fractional part of x?
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I don't understand where the "t" marked in purple goes
if its v_0t its added to the -1/2v_0t
how
There's a typo, should be v_0t instead of v_0
Ah it's been mentioned already
in my head that would be 0/2
thanks anyways though
but
if we put x to 5
then
-((1 ⋅ 5) / 2) + (1 ⋅ 5) = -5/2 + 5 = -2.5 + 5 = 2.5
ah
I see
yyyup
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how would you find the area of this ellipse
otherwise how could i convert it to standard reduced form (because the position doesn't matter)
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How to proceed?
first write this in terms of 2^(-1/x) and 3^(-1/x), for example 4^(-1/x) = (2^(-1/x))^2
After that...
ok so you should have 9 (2^(-1/x))^2 + 5 * 2^(-1/x) * 3^(-1/x) < 4 (3^(-1/x))^2 which we can rearrange to get 9 (2^(-1/x))^2 + 5 * 2^(-1/x) * 3^(-1/x) - 4 (3^(-1/x))^2 < 0
now if we set a = 2^(-1/x) and b = 3^(-1/x) do you see how this is kind of like (a + b)^2 = a^2 + 2ab + b^2, just our coefficients are wrong
@umbral wadi Has your question been resolved?
Ok
I got it now Thank you so much 🙂
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status 1
Can you predict what shape the line would be
imagine y=x^1000 vs y=x
Non negative coefficients are important here
Yes the function will be pointing up
imagine connecting 2 points , one is to the right and above the other
If you connected them with a y=x line, would that line be above a connection using y=x^2
Lowkey idk if I’m making sense lol
@tall moon
Do you know what a polynomial is
He definitely knows
its like the f(x)=x^something+x^something+...... right?
Yes so the highest degree of the polynomial will determine the shape of the line right
like x^6+666x^4+2x^3/3-4x+3 right
what are the shapes we can get
dunno i never played with polynomials on a graph before
i think like some sort of U shape and like a from down to up to down to up thing
like 2 candy canes stuck together on 0,0
yeah i forgot how to describe those shapes but you can get, either a line, a u shape, or the cubic x^3 shape
quadratic shape
yeah that goofy shape
Ok so we know that the function will hit a certain y value at a certain x value
I think I’m missing some vocabulary
Imagine how each shape would approach that point
dunno
This is kinda what I came up with
So the line is at the top the U is the middle and the cubic is the bottom
what is the difference in the shape of x^2 and x^4
like its more narrow?
Yeah it’s more narrow
Hmmm I don’t really have a way to formally prove it but I can imagine the solution
wait so to maximize f(12) a straight line is the best??
yes
o
If you imagine x^1000
the line would be very steep
So the y value right before hitting the point would be a lot lower
Than the linear graph whose slope is less
huh its still wrong
i got the graph 84x-480
at x=12 i get y=528
still wrong tho
wait a minute
its non negative coeficcients tho? @idle marten
a line has neg coefficients
@tall moon Has your question been resolved?
What part of the line function has a negative coefficient
what is a coefficient
like say ax+b, a and b are coefficients
for a line, the graph would be 84x-480 which has negative coefficients
that just means we need to increase the degree
you know I’m not sure what this topic is testing
I think you’ll end up with a quadratic where the line of symmetry is in the middle of the point tho
or a cubic function that’s weird
cauchy-schwarz inequaligy
ok i checked and the answer used C-S
thanks for your time tho
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oh so the limit as n goes to infinity should diverge right?
i uhhh
am not sure what i'm doing wrong
unless the interval can be something other than 9
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tl;dr are "for any" and "for all" interchangeable??
yes
just preference, it's our way of translating english into symbolic logic and back
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find $\tan \frac a2$ if $28 \cot a + 27 \sin^{-1} a = 3, a \ \epsilon \ (3 \pi; 4 \pi)$
nashira._.
$28 \cot a + 27 \sin^{-1} a = 3 \ a \ \epsilon \ (3 \pi; 4 \pi)$
nashira._.
Please show the original problem, exactly as it was stated to you, with the entire original context. A picture or screenshot is best. If the original problem is not in English, then post it anyway! The additional context might still be helpful. Do your best to provide a translation.
one sec
?
Lol
this is not arcsin it is 1/sin a
Bruh
That’s not how we usually write that..
well different places different conventions idk
coz in the book it is explicitly arcsin if they need it ig
Alr let's say it is 1/sin
yep
so I tried to convert the cot into cos a/sin a and the sin^-1 to 1/sin a and got
$\frac{28 \cos a}{\sin a} + \frac {27}{\sin a} = 3$
nashira._.
nashira._.
Quadratic it would be
You don’t need to do that
Yeh then just replace cos and sin with tana/2
yeah using the tan half angle formula
You can write cosA and sinA in terms of tan(A/2)
Yeah
To make it easy for eyes, use tana/2 as x
Look for the root which lies in the given interval
It's a/2 so keep that in mind
so how would we get tan(a/2)? Because yeah I get that we have the half angle formula. Would we find cos or sin or would we rearrange?
You don’t find either
Just write cosa and sina in terms of tan(a/2)
You can solve for tan(a/2) directly after that
$\tan \ \frac a2 = \sqrt{\frac{1 - cos a}{1+cos a}}$
nashira._.
so I rearrange for both cos and sin using the formula above?
No, not this
$$\cos a=\frac{1-\tan^2\frac{a}{2}}{1+\tan^2\frac{a}{2}}$$
kheerii
And $$\sin a=\frac{2\tan\frac{a}{2}}{1+\tan^2\frac{a}{2}}$$
kheerii
aaaah ok got it
yeah
ok so I've solved for them and I have gotten
$\tan \frac a2 = 5 \ \ \tan \frac a2 = -11$
nashira._.
the second one is the correct answer so my final question is
since it is in the period of 3pi to 4pi wouldnt they be both correct since tan is both positive and negative there?
,w 28(1-t^2)/(1+t^2) + 27 =6t/(1+t^2)
No
tan a could be positive and negative, sure, but we’re talking about tan(a/2)
If a is in (3pi, 4pi)
Then a/2 is in (3pi/2, 2pi)
ooooh shit sorry im just a dumbass 💀
thanks so much for the help
No worries
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When is
F( x ) = (x²+1)/(x²+3) Convex down
you wanna find where is it convex down or what ?
Yes
equate derivative to zero
graph of it
Yeah i know
So what's the domain when it's convex down
<@&286206848099549185>
Any help ???
Illiad
Yeah
It's when x < 0
F ' ( x ) = 0
When x = zero
Plug any number in the left of 0
F ' (x) < 0
Ok but the answer is wrong
what is it then ?
It says it's convex down from [-1 , 1]
i think the domain should be R
Me. Too
Illiad
So do you have any explanation for that?
Alright but the slope is negative for all x < 0
Then positive
Illiad
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ik im dumb but how do i find the gradient of these 9 secants it says i have to “use the equation to find the coordinate points and calculate the gradient” but idk what equation it’s talking about
. In the convex quadrilateral ABCD given below, m (∠BCD) = 90°, |AB| = |AC| and AC ∩ BD = K. If the areas of triangles AKD and BCK are 10 cm² and 25 cm² respectively, how many cm² is the area of quadrilateral ABCD?
A
10 cm
D
K
2
2
25 cm
B
C
A) 55
B) 60
C) 70
D) 105
E) 90
i think u have to find a new thingy channel cuz it’s occupied
the equation of f presumably
can you explain it in like dumb people terms because i’m only year 8 idk why i’m doing this work i’m confused
i assume you know the x values of the endpoints of the lines?
I know the start points but if i check with a ruler i can probably find the endpoint
can you show me exactly what was given to you?
if youre approximating tangents the way you doing it is very unhelpful
tangents are tangential to the graph, yours are just horizontally across
when it says to the right, it means that if one of your x values was 4 for example, the end points might be about 4.3, 4.5, 5
oh
somewhere such that the line between them is at least in the same direction roughly as the graph
oh so like this
wait let me try it
i’m confused on how i would draw a line like that while still intercepting the graph in 2 places
would it be like the pencil lines
thats better, yes
you may want to change the scale of your graph though, its a bit compact
yeah true but for some reason the range has to be -20 to 30
it says here in task 1 idk why
actually nvm i can just make the domain from like -10 to 10 so it’s wider
thank you that heped
helped
one last thing, what’s the difference between task 2 and 3 like what would i do differently
it’s saying you gotta do a different method but it looks like the same thing just the other direction
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What is the indefinite integral of 1/x²-1dx
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And of $\int \frac1{x^2} - 1 \dd x$, to make sure? Or did you mean $\int \frac1{x^2 - 1} \dd x$?
@upper sparrow
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cchhaart
Waterrrrbeeeeaaaammmmmm 





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in prism ABC-A1B1C1, Plane AA1C1C ⊥ Plane ABC, AB=AC=BC=AA1=2, A1B=√6
a) Proof: AC⊥A1B
b) Find the sine value of the dihedral angle A-CB1-B
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sorry i am new here
What is the question
here
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how do i solve this
because idk how to find part of a standard deviation
since mean + 1 standard deviation = 69 not 67
so idk how to find that
@vague grove Has your question been resolved?
I'd just use the normal cdf function on your calculator
Or a giant normal cdf table
(Please don't, we made calculators do it for a reason)
@vague grove
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I guess this doesn't makes sense right
1-2(1-sin^2)
= -1 +2sin^2
So π/4-4x/3 i treat just like variable or
yeah
$2\cos^2(x)-1=\cos(2x)$
Obotron
Got it
Got it
so what you have is equal to: $-\cos \left( \frac{\pi}{2} -\frac{8x}{3}\right )$
Obotron
but you probably want to use a different trigonometric identity like Obotron said
Now this seems...more appropriate I think
Bruh me and my camera
yeh that looks correct
@cloud flume Has your question been resolved?
Thabks
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to find the general solution
do i just take the determinant of this
with like lamda subtracted from the diag
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if you treat $x^3$ as its own variable (you can say for example $y = x^3$ if that's easier) then the polynomial is a quadratic and you can factor it as one
cloud
recognise by exponent rules that $x^6 = x^{3 \cdot 2} = (x^3)^2$
cloud
then we can make up some new variable, for example $u$, and say that $u = x^3$. then we have it that [ x^6 + 7x^3 - 8 = u^2 + 7u - 8 ]
cloud
that's the definition of a root, yes
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If $(x_n)$ is a sequence with $\lim x_n = a$, how do I show that $x\left(\mathbb{N}\right) \cup {a}$ is closed?
tales
Obviously every subsequence will have a as a limit
but it is not the case that any sequence in this set will be a subsequence of x_n
True
So consider a sequence in the set which converges to something not in the set, notably distinct from a.
Can the original sequence still converge to a?
@uncut thicket Has your question been resolved?
yes, because you could take a sequence of decreasing indices for example
it it not necessarily a subsequence
Well you can't decrease indefinitely
We're considering convergent sequences in the set
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what do i do here knowing the normals are not scalar multiples?
hwo do i know if they interesect?
well either we have that:
- the normals are parallel, so the planes are parallel, so they either don't intersect or are the same plane (their intersection is the entire plane)
- the normals are not parallel, so they are not parallel and do intersect (their intersection is a line)
so in this case its 2?
yes
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How can I solve this from examples as I never done this before
i get the question but idk what it means by "speed of the point of intersection"
theres two points of intersection
how is the point of intersection moving?
Just towards one another as tw points of intersection
Do you get it now
@proper marlin
I am getting a respone now on what is the meaning
The two circles are touching. Where the touch, that is the point of intersection. It would move up as the circles get closer.
Ok, so what I'd do is set the stationary circle to be $x^2+y^2=1$ and set the other circle to be $(x-2)^2+y^2=1$, so that way they are touching each other at exactly one point. Then, since the second circle is moving at 10m/s towards the other circle then the path of the circle is $$(x-2+10t)^2+y^2=1,$$ for $0\leq t\leq 0.2$ and so finding the point of intersection gives $$x=1+5t \quad \text{and} \quad y=\sqrt{10t+25t^2}.$$ Thus, the velocity in the $x$ and $y$ directions are $$\dot{x}=5 \quad \text{and} \quad \dot{y}=\frac{5+25t}{\sqrt{10t+25t^2}}$$ and so substitute $t=\frac{\sqrt{3}+2}{10}$ and find $|v|=\sqrt{(\dot{x}(t))^2+(\dot{y}(t))^2}$.
one eight seven
So then just input the t then for the equation
0.37
idk is that correct?
Okay
<@&286206848099549185> I need to chekc calulcations for this here
at t=(sqrt(3)+2)/10, they are sqrt3 apart
I got .37 and then plug it in to get 7.32
sqrt(25+(5.34)^2)
@proper marlin they describe it as wrong
<@&286206848099549185>
@proper marlin it was 10 m/s as was solved with a different method
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Call a permutation $a_1, a_2, \ldots, a_n$ of the integers $1, 2, \ldots, n$ quasi-increasing if $a_k \leq a_{k+1} + 2$ for each $1 \leq k \leq n-1$. For example, 53421 and 14253 are quasi-increasing permutations of the integers $1, 2, 3, 4, 5$, but 45123 is not. Find the number of quasi-increasing permutations of the integers $1, 2, \ldots, 7$.
Aurora
this sounds like a recursion problem
but im too stupid to think of the recursion
hints?
solved it nvm
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Someone can answer this?
@exotic umbra Has your question been resolved?
<@&286206848099549185>
assume there's 2 boys and 1 girl
then you can calculate the total masses of boys and girls, and add to get total mass of the class
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Hello. I have a question. Let's say there are five cards, and only one of them is winning card. You must pick a card randomly. If the card is winning card, you win and game is over. If you pick other card, you lose, put the card back, shuffle and give the deck to next player. There are three people playing this game, and each gets one chance to pick a card. What is the probability that the third player wins?
Now I have two answers and not sure which one is correct.
One is 16/125 (4/5 * 4/5 *1/5) and the other one is 84/625 (4/5 * 21/25 * 1/5)
16/125 is correct
the third player can only win if the first two players lose
since each time a new player picks a card, the card is added back and the deck is shuffled, each player's draw is independent
right. So I was confuses whether the probability of second person to lose is 4/5 or 21/25.
ok
But I need an explanation
maybe I need to ask this question in discussion.
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what have you tried?
Expanding the bottom two
what does that mean?
So xsquared + x
did you combine the fractions together?
the same way you subtract any other pair of fractions
by writing them with a common denominator
But how If x doesn’t have a common denominator
x doesn't even have a denominator at all
And how would that show a+b if I work out the fraction
what did you do here
it's the fractions that have denominators
Expanded
expanded what?
The denominator
just cross multiply
$\frac{a}{b} + \frac{c}{d} = \frac{da + bc}{bd}$
esca (@ with reply)
there's nothing to expand in the denominator here
i wouldn't call that cross multiplying
Yes bro, x and x + 1
so what did you get
are you talking about this
Yup
that's not expanding
you're applying it incorrectly
that's how you find the common denominator
But I didn’t now what to do
okay so
write the fractions with a common denominator
you agree that
$\frac{1}{x} = \frac{x+1}{x(x+1)}$
esca (@ with reply)
bros got a million helpers 
,, \f 1 x - \f 1 {x + 1} = \f {???} {x(x + 1)} - \f {???} {x(x + 1)}
you're supposed to do this
That is literally halfway from expanding
just show us what you have then
again, you're using the word expanding incorrectly
this is not "expanding"
this is rewriting the fractions with a common denominator, in preparation to combine them
how do you make x+1 into x(x+1)
Times by ohhh
Ok
I get it
But is that the answer
Bc it’s still not to square root
no it's not the answer
you still have to solve the equation for x
this is just working towards making it easier to solve
can you show what you have
U want me to resend the question
no i want you to show your working
can you combine the fractions now
Ye u get 1/x(x+1) = 4
do you know how to solve this equation
Nope
Ye I think
how would you do it
times by 1?
,, \f 2x = 4
like this?
Times by 2
you sure?
,, \f2x \by 2 = 4 \by 2
does this do what you want?
so now it's [ \f4x = 8 ]
No wouldn’t it be [ \fx = 8 ]
XVoid_76
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No wouldn’t it be [ \x = 8 ]
XVoid_76
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,, \f2x \by 2 = \f4x
this is how fraction multiplication works
like for example [ \f21 \by 2 = \f41 ]
I swear if u times idk wich the denominator or numerator but it would get rid of the other
Bc they are dividing each other so opposite is to times them
To get rid of the other no?
well yes
but you should remember which one is dividing which in a fraction
lest you need to go and review fractions
No I get how to do fractions
,, \f13 \by 3 = 1, \quad \f18 \by 8 = 1
so what is [ \f2x \by 2 ]
4/x
okay so back to the equation
I promise you do not
Hush
,, \f 2x = 4
how do you solve for x
Idk
you multiply by the denominator
just as we do here
,, \f2x \by x = 4 \by x
what does this become
So u get 2 = 4x
Ohhh so the denominator is what is being divided
i thought you knew how fractions worked
Brother
maybe you were lying
you're the one making these bold claims 
i just gave you one
No that don’t really count
ok... anyway
tell me what x is
U need to divide by 4 tho bc x isn’t on its own
yes so then x is?
0.5
damn not even a fraction
okay anyway that was just the warm up
back to the actual question
,, \f 1 {x(x + 1)} = 4
F fractions
how do you solve for x here
Times by x (x + 1)
no!
What lhs
anyway faiyrose feel free to take over i gotta dip
faiyrose
faiyrose
Oh that guy said differently
He said that he would be left with x(x+1) = 4x(x+1)
No
yeah that was you
you said that and then reacted to your own message with a checkmark
faiyrose
faiyrose
Yes
I have done it before
I can’t subtract 1 on the rhs bc they all have x next to them
faiyrose
Ok
Try and turn it into a + b squarerroot 2
Wich x needs to be on its own
I’m kinda lost at this point
Ye
I don’t remember it
But I know it
faiyrose
faiyrose
Hmmmm I wonder
faiyrose
$ax^2+bx+c+0=$
thewizardofOU
The numbers above it
Brooo I’m not that stupid
Come on look
4 x squared is axsquared
Bx is 4x
C is -1
Yes bro
faiyrose
Ok
How I get this
faiyrose
It’s it left like that
Not x (x+1) above and bellow
But how did u get that
Look this is my working out so far
But I don’t get how u then made the lhs a complete 1
I did
I made a common denominator
Oh wait
Hold on
I don’t change 4 yet until I need to get rid of the fraction huh
But still
I wouldn’t be left with 1
On the lhs
Bro I kinda gotta dip to school but thank u tho
Bye
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yo
sure
okay thank you
