#help-33
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how to find minimal solution (minimizing |x|+|y|) for linear diophantine equation (e.g. 17x+19y=524)
i have general solution to eqn:
x = 542(2)+11n
y= 542(-3)-17n
i just don't know how to find the n at which |x|+|y| is the lowest
Well the function |x|+|y| can be separated into at max three intervals since your equations for x and y are linear
|1084+11n| + |-1626-17n| is what you seek to minimize
so let's start by looking at the first term
when is 1084+11n positive and when is it negative?
yeah
so you have three cases:
-X-Y if n<-1626/17
-X+Y if n<-1084/11
X+Y else
if you find the minimum of these three you found the total minimum
shall we do the first one together
or do you prefer to attempt yourself
hmm
i feel like i've done smthn wrong
wait nvm i havent
im getting the total minimum = -1626/17
yop
i just used calculus for this tho
like first derivative test
i probably didnt have to do that
you don't need to
Its linear
which means the minimum of each interval
is one of the two boundary values
the minimum of a linear function on the interval [a,b] is either at a or b
true
wait i feel like i fucked this up tho
if i plug in -96
to my general solution it doesnt work ๐
maybe my general solution is wrong 1 sec lol
ok what was the initial question
kk
What was the initial question :D
nvm i got it
๐
@cyan cove Has your question been resolved?
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.close
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Does this seem to be right?
oh thank you
@lucid trail Has your question been resolved?
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Can somebody explain to me what the "0 rule" is? I just don't get it...
@quaint crescent is the "0 rule" the rule they used to break this up into cases?
x = 0 or x+3 = 0
Yeah
Ok
I translated it literally
Let's say you have two numbers
5 and 7
Let's call them y and z
Okay
And you know that y times z = 0
Let's just say that you arrived at this expression from something unrelated.
Or both
Right
But what about neither?
Then it wouldn't be possible
But with what logic does one cut apart x(x+3) into 2 seperate terms
Well yz = 0 implies y or z is zero. That is exactly y = 0 or z = 0.
Doesn't it imply that y*z is 0
So why is the multiplication thrown out and forgotten
There's some background stuff left untold or otherwise im just stupid
Well, I think you might have a rather rigid understanding of math manipulations, so it's understandable that this might make you uncomfortable. So let's approach it a different way.
You have x(x+3) = 0
Divide both sides by (x+3), we are asserting x+3 โ 0 by doing this.
x = 0/(x+3) = 0.
We can verify that 0+3 โ 0.
What about the case where x+3 = 0? Well in that case we have x(x+3) = x(0) = 0. Which from the original expression is a true statement. So x+3 = 0 is also valid.
You are not. Learning is hard
We asserted that x+3 โ 0. Using this assertion, we found x = 0. Now we must check that our assertion was not violated by substituting in the result into the assertion.
That's how we got x + 3 becomes 0 + 3.
Okay...
So when I see an expression where i have factored something
I can just take both of the terms and separate them
And make them equal 0
Yes.
Does the x+3=0 continue by turning into x=-3?
Yup!
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how do I work through these?
You have 2 points
of the tangent line
(0,0) and (b, f(b))
From that you can find the inclination
that is equal to f'(b)
i see, then i solve for b from the resulting linear graph correct?
yeah
how might I go about doing question 4b?
@safe sparrow Has your question been resolved?
got the first one, working on the second
if the derivative doesnt have k, how can i put k in terms of b using the following info?
so far ive worked out that the tangent line is y=x*2e^(2b)
and thus b = 1/2
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Completely 0?
@still temple Has your question been resolved?
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X is unknown matrix.
Determine X if we know XA = XC+B
Hint: Transpose and Gauss Elimination are your friend
@ebon depot Has your question been resolved?
@ebon depot Has your question been resolved?
XA - XC = B
X(A-C) = B
this tells you what the dimensions of X must be
So the dimensions of X multiplied with (A-C) which is a 2x3 should result in a 2x3 matrix?
yes
So X is a 3x3 matrix?
no..
yes
now you can either right multiply both sides by B^T or right multiply by (A-C)^T
I'm not sure what you mean
B^T=B transpose
AC is right multiplying A by C
so basically either X(A-C)B^T=BB^T or X(A-C)(A-C)^T=B(A-C)^T
now note the dimension of (A-C)B^T and (A-C)(A-C)^T in either case
But i'm trying to solve for X?
thank
FUCKKKKKKK chart is here
i've been trying to get help since 12 pm
7 hours and 42 minutes later ;-;
Awww 
Anyways, if we take the latter of these 
Do you know what the dimensions of these would be?
Sure, but if you multiply by one of those transposes, the answer to the previous question will give you a hint of what you can do 
one way of doing this, though that would be rather ugly, is to assume a general matrix X and then compute (A-C) and solve, but I wouldn't recommend that
@astral trout 
hiiii

i don't ge tit






calm down
no
What type of matrices are your favourite? 
Alright, say I gave you instead something like
[
\pmqty{2 & 1 \ 0 & 5} X = \pmqty{1 & 2 \ 3 & 4}
]
Would you know how to find $X$ in that case, or have any ideas?
@glass silo
Why does it have an inverse?
Sure, or because it's square 
yea
What are the shapes of these?
(A-C) = 2x3
B^T = 3x2
So then if you multiply them together, you get...? 
Ooohhh, a square matrix 
So it might be invertible 
If it is, we can just multiply by its inverse to get X by itself 
Merineth
since (A-C)B^T = 2x2 matrix?
Yea, that's a 2x2 matrix (as per above, it might be invertible, though of course it's worth making sure that it is - being square is necessary but not sufficient for being invertible!)
How does it become a 2x2 ?
oh nvm
Just tired
37 47
19 29
(A-C)B^T
and
182 22
22 62
BB^T

Looking kinda invertible 
annoying a bit
but doable
37 47 | 1 0
19 29 | 0 1
Don't i just divide by 37 on the top and 29 on the bottom to get 1:s instead of 37 and 29
and then i add with a mutliple of something to get 0 on 48 and 19
You could do it like that, but for 2x2 matrices, their inverses are quite easy to find
The inverse of $\pmqty{a & b \ c & d}$ when it is invertible is $\frac1{ad - bc}\pmqty{d & -b \ -c & a}$
@glass silo
ah right forgot
Okay there we go
that seems to be right?
inverted it to get it to the RHS
then did matrix multiplication
this is another question ?
Yes
so you have to find X is AX=3X?
"Determine all the vectors $X_{3x1} for which AX=3x if A = ... "$
Merineth
just a suggestion use \text{ the text }to enclose text when you're typing latex in the future
More that put the dollars only around the math
Yeah i know :( I've been at it for 8 hours and 40 minutes now and i'm on question 3 so i'm kinda lazy about latex atm
looks like a good method to me
thank GOD
3 more 
Hang in there mee!
do a inverse of an inverse cancel each other?
For example
I want to solve for A
Do i just take the inverse on both sides to remove the inverse on the LHS?
or sorry
I just remembered
If i multiply both sides with (A-6I) then LHS cancels out
You can though 
Help me :c
Can i do this
$(A-6i)^{-1} = M \
M(A-6i) = I \
MA - M6i = I \
MA = M6i + I \
A = 6i+I$
Is M supposed to be the matrix on the left, this one?
yes
And if so, no 
why
The right would be the identity matrix if you're doing it like that instead though, if you wanted
Multiplying an invertible matrix with its inverse gets you the (multiplicative) identity, not zero
i just get the identity matrix not zero?
But why can't i do it like i did?
and instead of zero
i make it I (identity matrix)
Merineth
@glass silo
what about this? expand the brackets and move over -6I or is this illegal too
Yea you gotta be careful, you don't necessarily have (A - 6I)^{-1} = A^{-1} - (6I)^{-1}
And apart from the last line, you could do it like that, the last line isn't quite like that @ebon depot 
A = 7i?
Not quite no
(also better to have all the small i's as capital I's, to represent the identity matrix)
What you would have, if M was invertible (it better be!) is A = 6I + M^{-1}
(multiply from the left by M^{-1})
what about it is wrong thjen :c
Explain what your thoughts are to get from the second last line to the last one?
how did you get to that equation?
i think i did it wrong??
no latex sorry i got beat up by c++ today
Remember you can't do $(A - 6I)^{-1} = A^{-1} - 6I^{-1}$, that's illegal
@glass silo
From $MA = M(6I) + I$, multiply on the left by $M^{-1}$ (as before it better be invertible!) to get $A = M^{-1} ( M(6I) + I )$
@glass silo
Expanding that out gets you $A = 6I + M^{-1}$
@glass silo
You can also notice that by inverting both sides of $(A - 6I)^{-1} = M$ to get $A - 6I = M^{-1}$ and $A = 6I + M^{-1}$
@glass silo
ah alright, so you're just inverting the expression on lhs and as such you invert the rhs as well?
and we can solve for A if and only if M has an inverse basically
i don't get how M appeared in this equation but maybe merineth will see it when she comes back
so if ur allowed to just invert the expression on lhs and do the same on rhs i feel like that would be the easiest?? if thats legal
Yep
you can invert both sides (it is legal to do that!)
From the MA = M(6I) + I part?
I take the inverse of M on both sides
$M^{-1}MA = M^{-1}M6I$
Merineth
you're missing stuff 
Merineth
See your last line here, everything you've done up until the last line is fine
$(A-6I)^{-1} = M \
M(A-6I) = I \
MA - M6I = I \
MA = M6I + I$
My initial thought was that i want to "move" M to the RHS
Capital I, not small i 
Merineth
So now finally i want to "move" M to the RHS so i solve for A
and to move a matrix
i need to multiply it by it's inverse, right?
So
$M^{-1}MA=M^{-1}M6I+I$
Merineth
$IA = I6I+I$
Merineth
WHAT
how is that wrong
i'm multiplying both sides with
M^{-1}
oh..
HAHAH
okay hold on
$M^{-1}MA = M^{-1}(M6I+I)$
Merineth
$IA = M^{-1}M6I + M^{-1}I$
Merineth
Merineth
Merineth

Don't you dare leave chartbit <3
got 2 more ;-;
just calculating the matrix for A
lmao

me holding chartbit hostage for math questions
What does it mean when a matrix is perpendicular and symmetrical?
orthogonal?
orthogonal matrix means its columns dot products are all 0
the columns form an orthogonal basis
Merineth
orthogonal matrices have a nice property
symmetric matrices also have a nice property
so this is saying A is an orthogonal and symmetric matrix?
Merineth
MSC2020 55N31 (Moosey)
@ebon depot
I'm not sure
$A^{2}=AA$
MSC2020 55N31 (Moosey)
Yea

no...
Merineth
I
The odd power makes it 21A ?
A, A^2=I, A^3=(A^2)A...
yes!
$21AI^{104}$ ?
Merineth
well, technically I^(52)
ah right
but still right
so now you can just apply inverse properties to find the inverse of B in terms of A
Merineth
$B = 100A$
Merineth
So now we just want the inverse of 100A?
ye
$B^{-1} = 100A^{-1}$
Merineth
Is this allowed?
note property 2
Merineth
Based on property 2
Nice
And since we aren't given the A matrix
i believe we are done?
Last one for today
and it's diiiiiiiiiiiiiiiisgusting
(The old man in the drawer) A cubic drawer has a corner at the origin and edges OA, OB and OC with other points in the points A = (2,โ2, 1), B = (1, 2, 2) and C = (โ2,โ1, 2). A dot-shaped old man is at the point with coordinates (0.9,โ0.9, 4.5). Is the old man in the drawer? Tip: Transfer to a suitable new base. Rephrase the question 'lie in the drawer' i algebraic terms. What finesse does the change to a new ON base offer?
if the point isnt as far away as B in the B direction, not as far away as C in the C direction and not as far away as A in the A direction then its in the cube
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Question is: find all 2x2 matrices A, with real entries such that A^2 = -I where I is identity matrix
Is this remotely correct
Oh the circled things are my answers
Im just wondering how youโd know how many different combinations there are as well
Ohhh I should have assumed bc = 0
So what are the form of the matrices ? Sry I didn't see the answer
What do u mean by form
If AD=0 we can't have bc=0
There is no AD, is there?
What are the matrices A such that Aยฒ = -I
The product ad
Because you Say a=0 and d=0
So the product is 0
Yeah but if the product isnโt in the matrix why is it relevant
Iโm not doubting u im just lost
Yes sure but we know that A is invertible so his determinant is non zero so with ad=0 we already know that bc is not 0
I don't read all the calculation but if you did that correctly it seems to be the good reasonning
Oh yeah I said bc = -1
To make it into identity matrix
Okay I donโt think this is wrong
But thereโs so many more answers ffs
๐
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quick SUVAT question
I have confirmed question a to be 1.22s to its max height then that will also be the same for time taken to reach starting point

that is correct
with part b would it be -1.5 x 1.22s or would it be for the full 2.44s
full 2.44
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can i please get help with this
i dont exactly see how they are going from first line to second
i mean i do get it visually when i take an example of 2 venn diagrams
but like if i were to show it mathematically
hmm ok
if A and B are disjoint, then P(A) + P(B) = P(A U B)
A and A' are disjoint
and so A n B and A' n B are as well
from there it's just undoing distributivity
we get something like this right
yes
the inside is equivalent to the inside in the original pic
it is undoing distributivity
how are we doing that i dont get that
(P n Q) u R = (P n R) u (Q n R)
oh
this
is the right side of this
identify P, Q, and R
then plug them into the left hand side
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I neeed help again
isnโt this kinda like physics
im confused here on what he did with the bounds to turn them into 2 and 0
from 20 and 16
looks like a usub
so, you either need to switch the bounds or keep track of the old variable
when you do a sub the bounds change
but its root x-16
oh, youre right
so if there are bounds and i have to do u sub then i need to plug in the bounds to the u sub formulas to see what they changed to?
yes
or, keep track of them
and undo the substitution on the back end
where did he get u=0 from
if you convert the bounds, you dont have to undo the substitution
Yeah
if you dont convert the bounds, you do
so you just have to decide which one is less work
where did he get u=0 from?
on the right side?
sqrt(16-16) = 0 yeah
so the upper bound becomes ur new x and ur lower bound becomes ur new U?
like why did he chose to sub in x=20 at the start and then u=16
did he mean if x=16 u = 0?
i think
@dawn elbow Has your question been resolved?
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Can anyone check if I've calculated this correctly? I guess this is physics but it seems appropriate to ask here
please ping on reply, will be heading off to bed ๐โค๏ธ
So equivalent resistance is (20R)/(20+R) + 11
Then V=IR so 13=0.35 times that mess
Then solve for R
@young swan
oh god wait is my shit wrong
So this ain't enough to just get the resistance for R ๐ญ?
That's a shame cause that is one beast of an algebraic question idk if i got the patience for
well, I was gonna head off but I kinda wanna solve this first.
2am math time? 
but btw will this work?
0.35A isn't like
the whole circuit, which i think was implied here
It's just through 20 Ohm resistor
Ohh wait im being dumb the amperage is out of the 20ohm
Not the whole circuit
Sorry im also really tired
It's alright this is one of three servers I've asked in, so even if one strikes out it's ok by me
You should head to bed if possible
I have 8 hours before i need an answer, bcs that's when my exam is. I'll be heading off to bed
AS YOU SHOULD !!!
โ
I'll be heading off legit rn, so once again, if anyone do reply, please ping ! โค๏ธ
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@spring stirrup Has your question been resolved?
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im understanding it cant be D and E because those would hacve the same magnitude
and i think its not A because the charges cancel from both things
and then my multiple choice brain gives me the answer B but it could be C and idk why it would be B
i think it would be B and not C cuz its closer to any charge
but if there was just an E and not a D i wouldnt know if it was B or E
and the second one im fully confused what its saying
@plucky blaze Has your question been resolved?
i think the 2nd one is A actually now bc there should be no force on that one itshoudl all cnacel out
@plucky blaze Has your question been resolved?
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hm wouldn't that still let me solve 2ยฒ = 2 + x + x^4 for x?
why 2ยฒ
are you one of those people that write R^+ for [0, infinity)?
since f(x) = sqrt(2 + x + x^4)
oh yeah right
I do both
wait nvm lol
yeah but how would I properly calculate it
can you get f'(x)
nup
what
am aware, but how would I determine x=1 in the first place :D
this
calculator i guess
oh so there's no direct approach?
theres no direct approach to solve this
but once you get x
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dibs
Do you see that angle 1 is equal to angle 3?
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Does anyone know to formula for question 11 b
@rigid breach Has your question been resolved?
No I donโt๐ญ
I donโt even know what a vertex is haha
Oh ok highest point
But idk how to find that
I think thereโs a formula but I completely forgot
you don't know what the vertext form is?
Nope
That reminds me of completing the square
Of a quadratic
But idk how itโll help in this case
what?
U said the vertex is h and k right?
So like, what do I do with them to get my answer
Cuz the answer is 159m
(h, k) is an ordered pair like (x, y) if that's what you're asking
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(The old man in the box) A cubic box has a corner at the origin and edges OA, OB
and OC with others in the points A = (2, โ2, 1), B = (1, 2, 2) and C =
(โ2, โ1, 2). A dot-shaped old man is at the point with coordinates
(0.9, โ0.9, 4.5). Is the old man in the drawer?
Tip: Transfer to a suitable new base. Rephrase the question 'lie in the drawer' in ยจ
algebraic terms. What finesse does the change to a new ON base offer?
I assume i want to make a base with OA, OB and OC?
(somewhat interesting how a different person posted this exact problem 2 days ago)
It's probably because we go to the same school and we aren't getting any help from the teachers on these questions
makes sense
OA/OB/OC define the shape so ye you want to transform them
@ebon depot Has your question been resolved?
Anyone?
I found a similar answer online but i dont' know how to interpret it
dead discord ๐
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what is this function
hii, im a grade 6 student and i need help with percentages convertion since its our test tomorrow, is tutoring allowed here? to be specific, I need help with decimal-fraction & vice versa, also fraction to percent. thanku!
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mod is the modulo function
$\mod(x,y) = x \mod y$
AlphaNull
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hii, im a grade 6 student and i need help with percentages convertion since its our test tomorrow, is tutoring allowed here? to be specific, I need help with decimal-fraction & vice versa, also fraction to percent. thanku!
ask in a new available help channel
how?
oh, thanks
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hello for this question, I always seem to get only on epossible answer for a and b (3 and 18)
i assume you plugged in a + ai into x^2 - 6x + b
you should have gotten $b = 6 a - 2 i (-3 a + a^2)$
AlphaNull
and using the fact that b must be real, its imaginary component must be 0
and the imaginary component on the RHS is a quadratic which has two solutions
i used the sum and product of roots rule
so we know the two roots are a-2i and a+2i so
a-2i+a+2i = 2a (sum)
(a-2i)(a+2i) = 2a^2 (product)
x^2-(sum of root)x + (product of root) = 0
x^2-2ax+2a^2
-2a=-6, a=3
2a^2=b
b=18
@drowsy gust thats whati did
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find g^-1(x)
No
f(f^-1(x)) = f^-1(f(x)) = x , u can verify urself using this
why +..
Alberto Z.
Because of this
show ur working
when u send 5 to lhs. It becomes
y+5
we should replace y place with 8x , no?
$$y = 8x - 5 $$
Adding 5 on both sides
$$y + 5 = 8x - 5 + 5$$
$$ y + 5 = 8x \cancel{-5} \cancel{+5}$$
JustToPro
$$y + 5 = 8x$$
JustToPro
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No, pay attention to the sign
When you square something you can't get a negative thing
xยฒ = xโขx
If x < 0, you get negโขneg = pos
If x > 0, you get posโขpos = pos
In both cases the result is positive
xยฒ is xยฒ, x is x
exponent stays same?
In fact xยฒ = x only for 0 or 1
Wdym?
like (-5)^2
Yes, that's xยฒ when x=-5
Yes exactly
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Stuck after this
Split 0 to pi/2 into 2 parts basically
No.
Same thing written with different limit
Is it sin(pi/4-x)
Yes
Sin (pi/4 -x) + (cos pi/4-x)
pi/2+pi/4 - x
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The matrix $A_{3x3}$ has the determinant, $det(A) = 2$ \
Calculate: $det(3A^2)$
Merineth
Do i form an equation or how do i go about solving this?
this 2 equation would help u i think
for n is matrix n x n
Hmm
Merineth
this?
that 2 equation is Properties of Determinants
using that 2 equation to solve ur problem
det(3A^2) = k^n (det(A^2))
Determinants has so many properties i sadly don't know them by heart
these 2 is 2 of the properties
These are the properties we are given
I assume the two you gave are the first two
Or nvm
They don't seem to be the properties you gave
for the Rakneregler or what ever
it prove this one
Ok!
for this is just say for the properties of power
Ex. det(A^2) = (detA)^2
Merineth
yes
any way we can apply a property to remove the 3 from inside the det?
ohh
det(3A) = 3^3 det(A)
$(3^3det(A))^2$
Merineth
$3^3(det(A))^2$
Flwo : 004
this one
why does 3^3 move outside the ()?
wait me a sec lemme get my book
for me ill say idk
my master just me teach me like that
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oh boy
@spark crystalSince you know determinants.
Do i need to find A^-1 here if i want to determine det(A^{-1})
if it help some way
what's the question?
Why am. I here
so find det(A) then do 1/det(A)
Funny enough he writes "Do you need to find A^-1?"
IMO applying some row transformations to get as many 0s as possible may be a good idea
I've never done det on anything greater than 3x3 matrix.
Does the method for 3x3 apply for 4x4?
which, the det formula or transformations?
afaik, the formula applies for a nby n matrix
I could be wrong though
This video explains how to find the determinant of a 4x4 matrix.
Introduction to Matrices: https://www.youtube.com/watch?v=yRwQ7A6jVLk
Adding and Subtracting Matrices:
https://www.youtube.com/watch?v=QXUbFzEd3Ww
Scalar Multiplication of Matrices:
https://www.youtube.com/watch?v=iJE...
I was thinking something like this
WIll take an eternity to calculate lmao
I think i have to do this first right?
Because trying to apply the method as he does in the video
will leave me prone for error
with so much calculations
Am i allowed to manipulate it as much as i want?
the 4th column are all multiples of each other?
I'm very bad at identifying stuff like that
the 4 elements on the top right are identical
I;ll show what I mean
just a mo
\begin{pmatrix}
1 & 2 \
1 & 2
\end{pmatrix}
Why am. I here
Compile Error! Click the
reaction for more information.
(You may edit your message to recompile.)
these elements
so if i take the first row * -1 and add it to the second row
or you could subtract the second row from teh first
uh, you want 0s in the first Row IMO
so do it the other way round, atelast I always expand a det wrt the first row, so that's what I'm comfortable with
Hmm
Afaik we wont get any 4x4 matrixes on the exam
so i probably will just use a online det calculator
This is a lot of calculation which i don't know lol
u should try to know
I want to but i've been at it since 10 am
i mean
mentally exhausted so i'm not sure i'll be even able to
it in my test at high school
Ok :)
yes
what's the answer?
I got mine to be 55

with that said...
