#help-36
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exactly what I said: explicitly show the values of s and q too
from what I can see, unless I missed some blindingly obvious gotcha, you only found p and r = -1
p+q+r+s=(p+q)+(r+s)=r+p
It should be -p = r+s
so finding s and q is not necessary
oh true
ah, fair enough then
unlucky
Luckily the answer is still -2
I'm just making sure you avoid any instance of people asking "but what about the other two variables?"
but if you have a solid enough reasoning around that, that's completely fair, and my bad for not realizing it
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i need help
1: random variable x has a probability density function of 3/8 (x^3-5x^2+6x) 0≤x≤2
A: sketch graph for this function
B: show that f(x) is a valid probability density function
C: to 3 significant figures find
i: its mean
ii: its mode
iii: P((1/2) ≤x≤ (3/2))
idk how to do part C at all, this was the question i was given
mean should be $\int x f(x)\dd{x}$ right?
〈 kitten | teacup 〉
i did i and iii, could i use the quadratic formula for ii? doing it rn seems like the right thing to do idk tho, dont have my textbook to check lol and ive gotta do this by tmrw
okay cool, thanks, you can like end the thingy now idk how to do it
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Sorry its not in english ill try to translate best i can
So basically a, b, c are any numbers that fit a + b + c = 1
and we have to prove the equation is correct
any numbers? or like for positive numbers
These problem seems like typical AM-GM type for positive number so I just want to check
yeah positive
you know how to solve it?
nahh, this type of problem has pretty unique sol for each
But you can predict when the equality hold and try to AM-GM
can u put a = b = c?
then u will get value of a that proves the equation a + b + c = 1 is correct
we are assuming
that a = b = c
and we will just a >= 1/3 after solving the equation
how do you know it so fast
?
no
ok lemme try
that is not how you're supposed to do a inequality problemmmmmm
but ehhh
just try it ig
solve it
i am
ok i got it
a => 0 and a => 1/3
ok well answer can be 1/3
but i strongly belive its incorrect for some reason
suree
this is last one i have unsolved
we have to find all combinations of a b c m n
and they are natural numbers (1, 2, 3, 4...)
substitute value of b in second equation?
what is substitute
putting value of something in another equation
so u get the value of b from first equy
and put it in second equi
hmm ok
so first square the expression a+b+c = 1
and find the value of the sum of their squares
then put them greater or = 0
then get an inequality using a.m h.m inequvalence

i did 1st
ok
lemme do the second
ok
this one
did you get how i did the first one though
dont give me solutions right away tho
ohh mb
no not really but ill try it later
do you want me to send u a soln ??
noooo
i want to work on the 2nd one ill try 1st one later on
with the square you said
damn smart asf
didn't think of that

can we try this
ok so what i think here
try getting value of b from first one
and from second one make c^m x c^n so we can also put in value of c^m from first one?
not sure how that helps in the long run
i think i got
find the value of a and b
in the terms of c
then focus on what you get for a
why value of a
it will be of the form c^m(5-c^n)/24
now it is given that all of them are natural numbers right
i am
from first one?
ok well that didnt help 🙁
how would you find a or b in this system of equations
forget the right side for now
b = c^m - 5a
yeah
what do you get then
now find the value of a and tell me
yea
find it in terms of c
yes
good
now in the bracket above multiply the -1 from the denominator
what do you get
there's a -1 below right
yes good
now take that - inside the brackets
believe
ok 🙁
now make cases
a = 5c^m-(c^n x c^m)/24
lets start by putting a = 1
good
now do you see 5c^m on both terms above
for some reason i assume it has to be interval as answer
take it common
i cant
its - and multiplication
i cant just do minus first
substraction

what
do you know 2X3+2X5 = 2X8

2x^3 times 2x^5 would be 2x^8
hm what else?
i still dont get what u want me to do
now we know a is +ve
wait a sec
and then apply the cases?
yes as a is positive so should the found expression
wait i think i understood
i got this yes
do you get what i did after that
now try to find combinations of values for c and n
waiiiiiittt
c^n < 5
5^1
nope
what other than that
nope just keep listing them
and n=1 as well for 2
ye
so c can be 1 and 2
n can be 1, 2 and any number
depends on scenario
well we get
c=1 n= any natural no.
c=2 n=1,2
c=3 n=1
c=4 n=1
right
yes that's why i included it
you said no
umm where
thats the short form for number
oh ok
no.=number
ok
now second task is to make a and integer
ohhh
so no fractions
what
now try to make A a natural number
input any?
by taking the case of c and n found above and using diffrent value's of m
so do i put a as 1
try it what all value's of m do you get
do that at the last
a = 1?
not really no
depends on n
use n=1 first
check if a value of m is possible
uhh no
yeah ok so i think well
both should have the same power
and anything greater than 3 right
yea
c = 3?
ye
how about c=4
no 4
a = 6.75 then
oh wait i can finish this on my own now
well i think with enough time i could have finished this one ty for help
so not possible
wlcm
4 is possible too btw
use .close
should we try 1st problem real quick?
if you have free time
i do
i mean i have 3 more problems but idk if i can do them all today
ok so
a + b + c = 1 would just be all squares = 1
square on both sides
1^2 = 1
and take 2ab + 2bc + 2ca the other side
you have a lot to learn
ok this is pointless for me then
ive never seen this expantion thing
ok anyways i dont think i could do this one so im done for today, its getting late so thanks for your time
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how do i do this
what is the small circle representing
what do each of those circle represent?
left is spanish, right is french
small circle is german
but german isn’t shaded in
wait what
why would german be the small circle
spanish would be small circle
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How should i start this, i dont have much understanding
do you know about the vertex form?
F(x) = ax^2 +bx + c
I just forgot how to implement it
Im reviewing for a test
that's not the vertex form
$f(x) = a(x - h)^2 + k$
kizzyyy
Damn
does this ring a bell?
Im washed
what does (h,k) represent?
Oh
the values of h and k here?
what would the new f(x) look like?
F(x) = a (x-5)^2+9?
X coordinate of vertex
-5
,, f(x) = a(x - (-5))^2 + 9
kizzyyy
anyway so
since -(-5) = 5
it's f(x) = a(x + 5)^2 + 9
all u have to do now is find the value of a
you'd use this particular fact to aid you with that:
"quadratic function.... passes through the point (-7,-15)"
Hi, I would like to know if question b requires a module, if so, how does this affect the result?
uhh
you should ask your question in #help-11
someone's already using this channel
Im sorry but
Okk sorry
How would i find this again
Dont worry about it
y = a(x + 5)^2 + 9
this passes through (-7,-15)
which means u can plug it into your equation and solve for a
y = -15 and x = -7
-15 but yes
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Why does this hold [
\ceil{\4ab} = \floor{\4{a+b-1}b}
]
do u know what those square brackets mean
I do, yes
can u simplify the right side?
by dividing it in three parts
yes, [
\floor{\4ab+1-\41b}
]
do u know pascal's identity?
i do not
it would be much easier with that ig
Looking it up, it seems to be a combinatoric identity; how does it apply in our case?
we get the right equation directly by applying the first one
lemme think of easier way wait
it always holds for a = b
I mean I managed to prove the case where a is a multiple of b
lex 
whats ur approach
what about if there is a remainder?
oh hi garlic! nice new pfp :D
ok yeah i think i figured it out
using the division algorithm, if $r = 0$, we have $a =qb$: [
\floor{\4{a+b-1}b} = \floor{q + \4{b-1}b}
]
and since $b\in\Z_{\ge1}$ that term will always be between 0 and 1, hence [
\floor{\4{a+b-1}b} = q
]
if $r\ne 0$ then [
\floor{\4{a+b-1}b} = \floor{q+1+\4{r-1}b}
]
Since $1\le r< b$ the term $r-1$ satisifes $0\le r-1< b-1$. Hence [
\floor{\4{a+b-1}b} = q + 1
]
well at least i think thats right?
yess
one question, how can you make sure r-1>0 are a and b integers or smth
@rugged merlin Has your question been resolved?
im using the division algorithm which only applies for integers a and b yeah
it says [
a= bq + r, \q 0 \le r < b
]
r being bounded that because, conceptually, division is just partitioning a number a into b cells, and r is the leftover component. If it was greater than b then it would mean the divisor could have fit in at least one more time.
But we're proving for real number no?
for my case i needed this for integers a and b, so not in my case no
also i mean i think you can come up with a counterexample that disproves it for the real case
,w ceiling(9.5/3) = floor((9.5+3-1)/3)
Ehhh I thought we're proving for real numbers
well it doesnt hold for that anyways so
Then I think your proof looks good. Or I'm not able to spot any flaw
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hello requesting help with this problem
the area of a sphere increases with 28 cm^2 per minute
with which speed does the volume of the cube increase when the radius is 6.5 cm
let me show my work
hol up
r is also a function of time
the area of a sphere increases with 28 cm^2 per minute
with which speed does the volume of the cube increase when the radius is 6.5 cm
volume of the cube?
What you computed was dA/dr not dA/dt
😔
it says the area of a sphere increases per minute
oh okey fine
so you wanna know the rate of change in terms of time, not distance
speed would more like be change in time
@harsh stratus Has your question been resolved?
why tf are you she/they
you are an it
a fucking it
!redir
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oh my bad dude i thought this was discussion channel because it was open by default. Sorry
Don't worry
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Hi im very confused on this proof
like
I understand that the cartesian product of A and B, order matters
I understand the idea of an ordered pair
but
Im unsure how the double power set comes in here
x in P(P(A)) means that x is a set of sets which are all subsets of A
yh this is mind boggling
so {{1}, {2}} is a set of sets each being a subset of {1,2}
ah yep, this makes sense
this is a formal definitions
how you want to remember AxB is as the set of ordered pairs of elements (a,b) where a in A and b in B
and all this proof really does is demonstrate how from the axioms we can build this set
so what the first power set does is, say for (1,2) u get {1}, {1,2} sets, the seocnd power set {{1}, {1,2}} ?
yeah
saying x in P(A) mean x is a subset of A
so x in P(P(A)) means x is a subset of P(A)
i feel like these without example would kill me
ahh i understand it now 
tysm
yea its kind of like builind on the sets
alr ty
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\def\blah{\xrightleftharpoons{\hspace{0.667cm}}} [ \int_{-\infty}^\infty g_1(t)g_2^(t-\tau)\dd t \overset{\mathcal F}\blah G_1(f)G_2^(f) ]
what does the left right harpoons mean here?
this is relating the time domain representation of the left function with the right which is the frequency Fourier domain representation
physicists ahh notation
Anyways, I am trying to make sense of what this is meant to be telling me
Well, its specifying a change in domain, = would be incorrect here
what is G, g ?
G is the fourier transform of g here
ahh ok and * is the complex conjugate ?
indeed
This is a cross correlation between functions i believe, whats your question exactly
understanding what "cross correlation" is, i suppose
In signal processing, cross-correlation is a measure of similarity of two series as a function of the displacement of one relative to the other. This is also known as a sliding dot product or sliding inner-product. It is commonly used for searching a long signal for a shorter, known feature. It has applications in pattern recognition, single par...
Its very similar to a convolution in function
It seems highly reminisicent of what convolution is to be honest
yeah
exactly
so there are three things: convolution, cross correlation and autocorrelation
Autocorrelation is like a convolution with itself
I mean the name literally means ‘correlation with itself’
Actually, the cross-correlation is precisely a convolution but it takes one functions conjugate
so cross correlation and convolution resolve to the same thing with real valued functions?
Well, the time domain is slightly different
I believe the time domain is negative over real functions
Thats how theyre related
I might be insane rn, lemme check something rq
Right, because the convolution is a function of t, but the cross correlation is of tau
So the time domain is negative for reals
They are otherwise the same here
We know this directly via a simple u sub
u=t-tau
But its important we keep tau and t consistent across the domains
This is probably where the confusion is
One sec
the thing on the right will have $\exp(j2\pi f\tau)$ as its kernel
uhh
the convolution should be put
$$(f*g)(t)=\int_\mathbb{R}f(\tau)g(t-\tau)d\tau$$
Cycadellic
is what i meant
compare with the cross-correlation
$$(f\star g)(\tau)=\int_\mathbb{R}f^*(t-\tau)g(t)dt$$
Cycadellic
yeah im trying to derive this formulation starting from convolution
so, take f's conjugate, convert to negative time domain, then they are related
start here, take the u-sub u=t-tau
aight
choose f^*, g for convolution
as opposed to f, g
apply convolution theorem, the result should follow immediately
,, \int_\R f(\tau)g(t-\tau)\dd t \overset{u= t-\tau}= -\int_\R f(t-u)g(u)\dd u
i think i did that correctly
now we want f^* instead of f
well do we conjugate twice?
what this is like
we want u here
yeah im saying u = t- \tau implies du = -dtau
then do convolution theorem here
Cycadellic
and so it is a modified convolution
oh i see
so applying the convolution theorem
wait how would that -tau affect our result from the convolution theorem
or would it
well, continue writing it out
what does the convolution theorem say about $(f^**g)(-\tau)$, now?
Cycadellic
wait but before that
im a bit unconvinced still with that formulation: [
(f^**g)(-\tau) = \int_\R f^(t)g(-\tau - t)\dd t \0{red}\ne \int_\R f^(t)g(t-\tau)\dd t
]
the latter being the actual correlation
@rugged merlin Has your question been resolved?
oh wait im literally going insane
LOL
theyre the same theorem here
but with G_2^* instead of G_2
no u-sub necessary
just swap t and tau, and take G_2^* instead of G_2
the u-sub with the negative sign made me lose my mind lol
😅
Still a bit confused by what convolution and correlation are really saying
I mean i get convolution but not this
this is very similar to the statistical cross-correlation with some extra assumptions
stats takes the expectation of it, is the real difference
i think this image on the wiki does a good job explaining exactly whats going on
well, hang on, the autocorrelation is on f\star f, and then g\star g
I see
Well visually
It seems like correlation is convolution but without the flipping that occurs bcuz of the g_2(t-tau)
specifically the correlation is a convolution, but with a sign flip and conjugate on the first function
over R, they really come out very similarly, except the convolution is commutative, which the cross correlation flips the domain on commutation
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you forgot to react to the bot
Yeah soz
What sign flip though
Ok wait
in the time domain
I think we need to establish something
we showed that $$(f\star g)=(f^*(-t)*g)$$
Cycadellic
Are these horizontal axes t in both?
Is this -t reallt correct though in hindsight
It just doesn't check out with the convolution theorem
But im eyeballing it rn
i dont really think this distinction is particularly significant in application tbh
use your help channel, this one is occupied
Oh ok
Well i mean im asking bcuz im trying to figure out what those axes in thr images are supposed to be ig
they are both time domains, we should probably think of the axes as the same there
Correlation seems to be just 1. Flip t and tau 2. Chug a * from the conv formula
So i dont know where the -t here is coming from
$$(fg)(t)=\int_\mathbb{R}f(\tau)g(t-\tau)d\tau$$
let $u=t-\tau\implies du=-d\tau$
$$(f^(-t)g)=-\int_\infty^{-\infty}f^(u-t)g(u)du$$
and so, $$f^*(-t)*g=f\star g$$
because u = infinity implies tau = -infinity
so thats how we deal with the minus sign here
@rugged merlin Has your question been resolved?
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hi im a bit confused, if i have a matrix m x n why is it that:
if m > n, there is no way that it's surjective?
and why is it that if m < n, there;s no way it's injective?
i wanna understand the logic behind i t
i would say it’s easier to understand that logic if you know that any matrix represents a linear transformation
are you familiar with that idea?
If m>n, the matrix maps from a smaller space to a larger one. There aren’t enough independent directions in the input to reach every point in the higher-dimensional output. So it can’t cover the whole output space, meaning it’s not surjective.
matrices/linear maps cannot grow dimensions
it can only keep its current dimension or reduce it
yeah
i still dont really understand this idea..
o so
if i have r3 --> r2
then it's surjective
but if it have r3 --> r3 it cannot be?
well it can still be surjective
because we have an input space of dimension 3 and an output space of dimension 3
there would exist some linear map that would retain its "size"
Only if invertible
but take T: R^3 -> R^4
theres just no possible linear map you can make such that the codomain is all of R^4
thats the idea of how linear maps cannot grow dimensions
@solar crest Has your question been resolved?
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Hi. I'm not sure how to use the hint in this question, can anyone help?
The function $\delta(x)$ refers to the dirac delta function. That is:
[
\int_a^b \delta(x)dx = \begin{cases} 1 \quad \text{if: } 0 \in (a, b) \
0 \quad \text{otherwise.}
\end{cases}
]
Good Luck HSC Students!
did you look at integrating delta(ax)?
I did take the integral and set x = au, where do I go from there though?
Or did you literally mean integrating delta(ax) straight from the start?
well with the test function
I'm not sure how you would do that sorry.
$\int_a^b \delta(ax) \phi(x) \dd{x}$
knief
u = ax
@main mirage Has your question been resolved?
Hmm. Still a little unsure, but this is what I am thinking of:
[ \int_{-\infty}^{\infty}\phi\left(x\right)\delta(x)dx=\phi\left(0\right) ] [ \int_{-\infty}^{\infty}\phi\left(au\right)\delta(au)adu=\phi\left(0\right) ] [ \int_{-\infty}^{\infty}\phi\left(x\right)\delta(x)dx=a\int_{-\infty}^{\infty}\phi\left(au\right)\delta(au)du ] If we let $\phi(x) = 1$, then: [ \int_{-\infty}^{\infty}\delta(x)dx=a\int_{-\infty}^{\infty}\delta(au)du ] Is this on the right track?
Good Luck HSC Students!
I'm guessing there should really be |a| at the front of the integral, because when we make the substitution x = au, we don't want the bounds of the integral to flip?
However, does this mean the integrals cancel out? What is the justification for that?
@main mirage Has your question been resolved?
@main mirage Has your question been resolved?
"good luck HSC students" is a great message
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lemme send the hints
First thing i think of is euler line, draw centroid
599=||This is just a statement about distances to line OH; ignore the areas.||
152=|| You can replace line OH with any line through the centroid G.||
598=|| Without loss of generality, B, C lie on the same side of the line. Let D be the midpoint of BC.||
545=||Use the fact that AG = 2GD.||
yeah im stuck on how i would prove AX=BY+CZ
i havent used the last hint yet so its prolly related to it
pigeonhole principle - two sides of the line, 3 points so two points are on the same side
yeah i just realized that
huh wait ok hold on this is interesting
DK is half of AX
since BY//DK//CZ, and D is the midpoint of BC, by similarity DK=(BY+CZ)/2 <=> AX=BY+CZ, QED?
ok yeah i think this is it
thank you so much!
.solved
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Over here, is the purpose of finding a general equation?
I dont understand what we are exactly doing here, the idea is to use differential equations:
Sorry if the handwriting is bad
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Hey, extremly rough draft as i just learned little o notation 5m ago but is any of this correct
Im mostly asking for the potentially botched use of some sort of triangle inequality with h1, and h2
forgot to say by using squeeze theorem we can bind g(h) between 0 and 3h at the end there **
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hi guys, dumb question
This, but with n = k+1
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yeah, some rando spammed some scam ss's
lotsa those these days. 
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$\int_{0}^{1} \int_{0}^{1} 2x^2y+Cy^5=1$
\
Solving this will give me the value of C
the joint PDF doesn't involve integrals..
over (0,1)*(0,1)
wai
yep
I integrated 2x^3+Cxy^5
haha numbers are hard
😭
I did a worse mistake in my probablituy midsem
$\int x \cdot 2x^2 dx = x^2$
wai
😭
yea stupid calculus mistakes are the source of many problems in higher level courses, alas
Sorry, more than help, I need to rant
nw, it happens to all of us
gets better, while calculating the product of 2 cycles I read 2 as 1
and while calculating something I got 30 instead of 35 ( cost me 5 marks)
harsh grading!
haha , see hlounge for harsher grading
anyways, just to be sure
this kind of crap is why i always stayed until the end of every exam even if i finished early
double and triple check everything, i nearly always found some stupid arithmetic mistake
here once I get C, I integrate wrt y from 0 to 1 fo get F_X(x) and similarly for F_Y(y)
Bungo
so you would integrate from x = 0 to 0.8, and y = 0 to 0.6
I also need F_X(x) and F_Y(y)
oh ok i overlooked that, lemme go reread
yea assuming you meant f and not F in both cases
(this gives you the marginal pdf's, not the marginal cdf's)
yw
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its on german so dont wonder. My question to this is what does it means when s = -1.5? like a = -1.5c, but what does that means ?
${\vec{a} = -1.5\vec{c}}$
k
it means that vector a is in the opposite direction of vector c
and its magnitude is 1.5 times that of c
yes
and is pointing in opposite direction
yup
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used an translator and idk if its the right topic
lines in space,
the vector parametric equation
anyways, my question is how do i use that with an line in an 3d coordiante system?
die vector gleichung von der linie die durch A und B geht? sorry mein deutsch ist nicht so gut
beim ersten bild wird das ganze ja mit einem 2d koordinatensystem gemacht also mit x und y.
ich vestehe aber nicht wie ich den vector AB bei x y z herausfinde, da man ja bei nur x y ablesen konnte
ich verstehe nicht wie die das mit 7, -1, 2 herausgefunden haben
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I dont get how they solved this one
I understand that without the restriction its 54^5
yes
we need the restriction and the 26^5 gets rid of the number of passwords with only lowercase
cuz 54^5 includes the only lowercase and the mixed lower-upper
yes
yep
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np
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Write the pseudocode for an algorithm that determines if a function is injective, assuming that you can iterate through all the elements in the domain and the co-domain in a finite number of steps.
Im in a Discrete math course following Kenneth Rosens Textbook for discrete math
and im just wondering how should i refer to the co domain in pseudo code
should i just reference it, like b1,...bm: corresponding elements of co-domain?
Thats just up to your liking tbh given its pseudocode
If we assume f: A -> B
A = {a1,a2...an}
B = {b1,b2...bm}
is intuitive enough
ok
so could i technically also just refer to the output using f(a1)?
in my algorithm
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If you remember the general idea of Surjection and Injection this one is pretty easy
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ive formed DEs $\frac{dx}{dz} = \frac{-(2x + z)}{x}, \frac{dy}{dz} = \frac{2y}{x}$ but stuck form there
ashyboi
They should be d/dt
anyways, this is more like solving 1 diff. equation and 1 diff.system
how come btw
i kinda just assumed the flow was steady state
and there was no t dependence
We basically want a function for the position, right?
yes
Velocity is the derivative of position through time
So here we got this:
$\u = \frac{dR}{dt}=(4x+2z,-4y,-2x)$
∫ᴄ 𝐅·𝑑𝑟 = ∬ʀ ∇⨯𝐅 𝑑𝐴
where R is the position
Or, in another words
Here you can see that x and z depend on eachother
and y depends on itself
Soooo, they are basically
1 system
1 equation
From solving those two separately, you can combine them back into vector notation
ye i see it
tbh, a lot of physics problem are usually written down through dt
They did

