/sqt/ - Stupid Questions Thread - Shitty Bait Edition

Tips for good questions:
>provide context
>show partial work
>check stackexchange.com and wolframalpha.com

Previous thread:

homie there's already one here

>posted before the bump limit

is sexuality a spectrum?

no. calling it a "spectrum" implies there's a linear range of values. sexuality is more complicated than that.

Veeky Forums is one slow board

Why won't math get me money?

it will, if you use it for programming or engineering or something

What's the most rigorous linear algebra textbook?

Can someone explain to me what's going on in this proof by induction?

I got the first couple of steps correct, but my proof has about 5 extra steps, I didn't know it could be done in 2 steps.

rejected by kek. besides your identity and sexuality should be things you are least concerned with. what you need to be concerned with is your spot in Heaven.

lang algebra. if you were serious about learning you would have known this

>lang algebra
this?

First step is separating the k+1 term from the whole sum
Second step is applying the inductive hypothesis to the summation
Third step is seeing that F_k+2 + F_k+1 = F_k+3 by the definition of the Fibonacci sequence (add together the 2 previous fibb numbers to get the next)
Last is just realizing that F_k+3 - 1 = F_(k+1)+2 - 1 which finishes the proof

Linear Algebra chapters of Aluffi

You know how Superman can fly around the Earth so fast it reverses the direction it spins in? Would that be possible in real life?

Hey I am an EE student about to be junior in the fall and need to know if it is too late to get into top grad schools by now. Just realized I wanted to do it recently, but have no real research yet. Could I possibly get into top schools like UMich, UIUC, Cornell, etc. with only about 1 year of research?

Forgot to add: I plan to do research summer 2018 for sure and hopefully through the school year 2018-19. Might even spread my last credit hours to stay another semester in fall 2019 to get more research during summer 2019. So at best I'm looking at 2ish years of research. gpa is 3.97 so I am fine there.

thank you

i always feel like i'm stumbling around in the dark with proofs, even at this basic level. i will get better by just trying more proofs though, right?

Yeah, for the most part, you just need more experience with the tricks used in proofs and proof writing in general. Most of the time difficult problems just require more critical thinking, analysis of the problem, or an overall change in thinking. Sometimes taking a break and going for a short walk can help to redirect thinking.

How can i calculate area of rectangle using coordinates?
I have position of bottom left and top right
btw, cant do something like 4-2 to calculate

(4-2)*(4-2)=2*2=4

read last line i typed
>btw, cant do something like 4-2 to calculate

but you can, i just did it

What is btfo and why isn't it in the wiki search results? Seems important to know

Lurk more

Suppose you have two coordinates which are the top left and bottom right of a rectangle (x,y) and (a,b) respectively
Then the area of the rectangle is (a-x)*(y-b)

Can you integrate y=4 over [2,4] then subtract y=2 over [2,4]?

This is a super scrub question, but I can't solve this one. I can't just square both sides since that would be assuming what I'm trying to prove.

If I managed to manipulate the equation to somehow be 0 = 0 that wouldn't show it to be true either, since P -> Q having Q be true doesn't show that P is true.

Can anyone help?

do you have a set of axioms you have to work with?

sqrt(a/b) is by definition the non-negative number which squares to a/b, so if you show sqrt(a)/sqrt(b) satisfies that you're done

Isn't this just true by just by the properties of exponents? E.g. (ab)^k = (a^k)(b^k)

If this isn't what is expected, maybe try contradiction

Sorry, I was being retarded and not looking at the definitions given. I *can* simply square both sides, but not in the way I was originally thinking (multiplying both sides by each other), but by this definition.

Here's a related question I have about proofs like this in general. To solve this one I can easily just multiply 1 / (1 - sqrt(2)) by (1 + sqrt(2)) to find that it gives the right side. That's nice.

HOWEVER, what I'm wondering is a method where I multiply both sides by (1 - sqrt(2)) and then finding that 1 = 1, which is true, also prove it? I.e. can I prove stuff like this in the same way I solve equations, or am I assuming the truth of what I'm trying to prove here? Or is arriving at a truth (Like 1 = 1) from a supposed truth (Like that in the picture) a proof of the supposed thing?

I know it's a bit of a convoluted question, but it's been bugging me for a while.

>HOWEVER, what I'm wondering is a method where I multiply both sides by (1 - sqrt(2)) and then finding that 1 = 1, which is true, also prove it? I.e. can I prove stuff like this in the same way I solve equations, or am I assuming the truth of what I'm trying to prove here?
no you never start with what you're trying to prove, mainly because once you've assumed that there's no more work to be done, but also just because you arrive at a true statement doesn't mean what you started with was true since some operations aren't reversible

i.e. pretend the question said 'prove that 1/(1-sqrt(2))=-(1+sqrt(3))' instead, you could multiply both sides by 0 to get 0=0 but the initial statement isn't true

also logically
>multiply both sides by (1 - sqrt(2)) and then finding that 1 = 1, which is true
only lets you conclude that IF 1 / (1 - sqrt(2)) = -(1+sqrt(2)) THEN 1=1, it doesn't say anything about whether 1 / (1 - sqrt(2)) equals -(1+sqrt(2)) or not

Thanks my dudes, that's like having a stone lifted from my chest. It's been bugging me for so long, and it never felt right.

I think I got confused because I was seeing questions in the form of If P prove Q changing both sides, which is allowed (right?) and confusing them with questions like these.

The "multiply both sides by 0" example was really good too!

>I think I got confused because I was seeing questions in the form of If P prove Q changing both sides, which is allowed (right?)
yep

i'm not sure but maybe because he doesn't seem to obey newton's third law of motion with the way he flies and if he could maintain a very high speed like close to light speed that would take a tremendous amount of energy and he would push a lot of air

nah

earth has rotational energy of [math] 2.9197 \cdot 10^{33} \space J[/math]

the tsar bomba (strongest nuke) has an energetic release of roughly [math] 2.1 \cdot 10^{17} \space J [/math]

as you can see it would take something [math] 1.39 \cdot 10^{16} [/math] times stronger which isn't exactly realistic

forgot to mention that's only to stop the rotation of the earth assuming 100% transmission of energy into stopping it

Please? I mean I do understand the meaning but everywhere I look for the expansion I get a different answer - bitch the fuck out, bounce the fuck out, etc. Please help a toddler achieve greatness

Can someone please explain why these answers are the correct answers?

[eqn]\forall x \forall y\left(\left( \sqrt\frac{x}{y} = \frac{\sqrt x}{\sqrt y}\right) \rightarrow \exists z \left(\left(z = \sqrt\frac{x}{y}\right) \rightarrow \left( z = \frac{\sqrt x}{\sqrt y} \right)\right)\right)[/eqn]

Really stuck on this one.

I started by writing A(Bej) but then i am at a loss.

Starting pre calc. I understand what absolute value is but i dont get this. absolute value is always positve, right?

>absolute value is always positve, right?
absolute value is always non-negative (i.e. |x|>=0)

HOW THE FUCK DO I DO THIS FFS?!

calculus brainlet

Hey Veeky Forums I'm working on Fourier transformation problems, and in the solutions to one of the problems the following simplification is done.

Can anyone explain how the complex component j ends up in the denominator?

I can see the exponentials have changed place: Before it was exp(-j) - exp(j) and after it is exp(j)-exp(-j), but how does this add the complex j term in the denominator?

enjoy your ban useless faggot :')

Bruh, calm the fuck down, it is not that hard.

Here is what you have to do: take the first derivative of that function and then set it equal to 0.

Do some trigonometry to solve the equation you previously got to 0.

After that, again use some trigonometry to find exactly how many of those solutions lie in the interval (-1,3)

If at any step of this you get stuck you can google or if you want you can report back with your progress and I'll help you a bit more.

at least i know babby's calculus brainlet :')

-j=1/j

Thanks man, I was stuck for hours.

I get sin2x = 1/2 which has only TWO SOLUTIONS FFS. The answer is 5.

I'm assuming it's because the spots will just spill out into the solvent and because ink is a liquid that will travel up the plate whereas pencil lead won't.

That derivative is very, very wrong.
Research the chain rule and the sum rule.

might want to learn how to take a derivative brainlet

do you not even know how to check when you're wrong with wolfram alpha yet? you shouldn't be looking for extrema if you don't know how to derive

...

It's not just derivative, I also did trig transformations ffs. Though I've figured out where I went wrong already. I divided by cosine.

>sin2x = 1/2
>only two solutions

babby here
what is the point of universal algebra if we have category theory

Hint that will serve you in the future:
When you are looking for solutions you should never ever ever divide by anything that could potentially be 0 (like cosx). Because when you divide by those things, you remove information for when the thing you divided could be 0.

For example: Imagine if you are looking for the roots of the simple equation x=0
And then you divide by x, that leaves you with 1=0, which "tells" you there is no solution. But that is wrong.

>what is the point of universal algebra if we have category theory
because universal algebra is the study of algebraic structures while category theory is the study of categories, they're two different things

I'm using open office and I've made a table.

I'm counting stuff and write a 1 for every item I see so in the table it looks like

11111111 (etc)

BUT open office converts it to some thing so for example 26 "1"s become 1,11E+025

Now what does that mean? And how do I get it back?

On (-1 to 3) ffs read the fucking posts.

I'm bumping cus I'm desperate guys I need to resolve this asap. I've got several pages of info now that I can't use since I didn't notice the change until now.

>counting in base 1
why though?

Have you tried formatting the cell as text?

It was close on the keyboard, I know I should have probably used like x or something but what's done is done *cries* how do you format it as text?

I just need to know why 26=1,11E+025. If I can figure that out I could probably figure out the rest of my stuff too.

right-click cell > format-cells > far left tab on the format-cells window named "Number" or "Numbers" or something > should be easy enough to find after that

Also you may have to convert to and from numbers and text.

Clearly.

Here's what I get

1 - 2sin^2(x) - 4sinxcosx + 8sin^3(x)cosx = 0

WTF CAN I DO HERE FFS?!

I understand that by multiplying the expression with -1, I can utilize [math] -j = \frac{1}{j} [/math] to get j in the denominator.

But won't the negative exponential always leave me one positive j component?

See:

[math] (-1) \cdot (\frac{j}{2} \cdot (e^{-j \cdot \omega_{0} \cdot t} - e^{j \cdot \omega_{0} \cdot t} )) [/math]

I will get:

[math] - \frac{j}{2} \cdot (e^{-j \cdot \omega_{0} \cdot t} - e^{j \cdot \omega_{0} \cdot t} ) = - \frac{j}{2} \cdot e^{-j \cdot \omega_{0} \cdot t} + \frac{j}{2} \cdot e^{j \cdot \omega_{0} \cdot t} [/math]

Or am I supposed to say:

[math] (-1) \cdot (\frac{j}{2} \cdot (e^{-j \cdot \omega_{0} \cdot t} - e^{j \cdot \omega_{0} \cdot t} )) = - \frac{1}{2j} \cdot (e^{-j \cdot \omega_{0} \cdot t} - e^{j \cdot \omega_{0} \cdot t} ) [/math]

But then how can I choose this and not the other? Do I decide based on what makes most sense in the expression?

That is either not the derivative, or it is not completely simplified.

Use wolfram alpha to get the right form of the derivative man.

lmao how can you be this bad at taking a derivative brainlet? please show your steps so i can laugh at how badly you got it this wrong

the second one ((−1)⋅(j/2⋅(e^(−j⋅ω0⋅t)−e^(j⋅ω0⋅t)))=−1/2j⋅(e^(−j⋅ω0⋅t)−e^(j⋅ω0⋅t))) is wrong, you didn't get rid of the negative when you moved the j into the denominator

>Really stuck on this one.
>I started by writing A(Bej) but then i am at a loss.
By definition of product:
[math]C=AB[/math]
[math]C_ij=\sum_k A_{ik} B_{kj}[math]
[math]A_{ik}=a_k[/math] by your definition of the last. Finally
[math]Ce_r=C_{ir}=\sum_k A_{ik}B_{kr}=\sum_k a_{k}B_{kr}[/math]

No fucking shit it's not completely simplified. The question WHAT ELSE CAN YOU DO HERE FFS?

You fucking retard the derivative is
> 2cos2x - 2sin4x

That much is fucking clear.

Question is WHAT CAN YOU FUCKING DO?! It has to be either parenthesised out or only same function of the same angle. BUT IT DOESN'T GET THAT WAY.

>WHAT CAN YOU FUCKING DO?!
set it to zero and solve for x brainlet, is this your first finding extrema problem?

>the second one is wrong, you didn't get rid of the negative when you moved the j into the denominator

True, thanks.

The thing is though, that if I get rid of the negative by putting j in the denominator, then I can't change the signs of the exponentials.

So then how the heck do I end up with this expression:

you seem a bit confused but here:

(j/2)(e^{-jwt}-e^{jwt})
= (-j/2)(e^{jwt}-e^{-jwt})
=1/(2j) (e^{jwt}-e^{-jwt})

You idiotic fucking shithead HERE'S WHAT I FUCKING GET WHAT CAN YOU DO ____HERE____ FFS?! Clearly fucking nothing since all you do is just shit talk like a 5 yo faggot.

Ah okay, so you just switch the sign of all the components in the expression.

This makes more sense than to multiply it by (-1).

Thanks a million!

but setting that expression to 0 and solving for x doesn't throw in all those other terms brainlet, why do you overcomplicate things with useless trig identities?

don't you know what sine and cosine look like brainlet? just draw a picture and you'll see the five points

I THINK I GOT IT FFS

> 4sin(x)cos(x)cos2x = cos2x

www.wolframalpha com/input/?i=4sinxcosxcos2x+%3D+cos2x;

BUT WHAT THE FUCK DO I DO NOW?! HOW SOLVE THIS SHIT?

FFS you're right, thanks.

So anyway what's up with this fucking shit?

There's two negative answers: -1 and -1.5. YOU CAN FUCKING GUESS WHICH ONE IS THE BIGGEST.

And yet they say it's -1.5

Is it just them being fucking retarded illogical cunts? Or am I missing something this time?

When doing trig substitution what happens to dx and boundaries?

For example.

INTEGRAL^2_1(sqrt[2(1 - (x/2)^2])dx = { x/2 === sint} = INTEGRAL^?_?(sqrt[2]cost)d?

Re-asking as no one answered me last thread.
I want to get far in optimization (in real domain).
Currently I have studied real analysis, convex optimization.
I'm thinking about getting to work on some differential geometry books.
Will they help?

doesn't matter what kind of substitution it is, for any substitution you get the boundaries the same way, plugging it into the substitution

i.e. if x=1 then sint=1/2

So it would be INTEGRAL^(pi/2)_(pi/6)(sqrt(2)cost)dt?

Blown the fuck out
now fuck off

>derive
I hope you know that differentiation is different

of course, but people still understand what you mean with derive

How do I get the volume of this cunt? I got the area, so do I just integrate it and that's it?

Can you translate to english to give more context?

Volume of revolution? if so search for disk method and shell method. This kind of stuff has very nice visualizations

cause that's clearly not fucking it

What is recommended book on algebraic number theory?

>Pennsylvania University, Philadelphia
>Mellon University, Pittsburgh
Out of these two options for ENGINEERING, which one do you think would give me a higher chance of working in Japan? Provide reasoning and proofs for bonus points.

Do some research on whether they have exchange program with a Japanese uni.