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/sqt/ /qtddtot/ - stupid questions thread

Anonymous Tue 02 Mar 21:49:14 2021 No.12777391 View Reply Original Report

Quoted By:

previous thread: >>12753409

>what is /sqt/ for?
Questions regarding math and science, plus related advice requests.
>where do I go for other questions and (advice) requests?
>>>/wsr/ >>>/g/sqt >>>/diy/sqt >>>/adv/ etc.
>how do I post math symbols (Latex)?
https://rentry.co/sci-latex-v1
>a plain google search didn't return anything, is there anything else I should try before asking the question here?
https://scholar.google.com/
>where can I look up if the question has already been asked here?
https://warosu.org/sci/
https://boards.fireden.net/sci/
>how do I optimize an image losslessly?
https://trimage.org/
https://pnggauntlet.com/

>where can I get:
>books?
https://spoon.wiki/Books
https://stitz-zeager.com/
>articles?
sci-hub.st
>book recs?
https://sites.google.com/site/scienceandmathguide/
https://4chan-science.fandom.com/wiki//sci/_Wiki
http://math.ucr.edu/home/baez/physics/Administrivia/booklist.html
>carreer advice?
https://sciencecareergeneral.neocities.org/
>help with calculus?
https://spoon.wiki/WolframAlpha
>charts?
https://imgur.com/a/JY6NNeL
https://imgur.com/a/0qDEgYt
>tables, properties and material selection?
https://www.engineeringtoolbox.com/
http://www.matweb.com/

General advice for asking questions here:
>attach an image (animal images are best. Grab them from >>>/an/)
>avoid replying to yourself
>ask anonymously
>remember to check the Latex with the Tex button on the posting box
>if someone replies to your question with a shitpost, ignore it
>avoid arguing with Yukarifag
>do not tell us you came from whatever the fuck board, /pol/ in particular
>do not mention how [other place] didn't answer your question so you're reposting it here
>If you use j for the complex unit, put a ¿ somewhere in your post or use emoticons I will automatically ignore your question. I don't actually know about everyone else, but you shouldn't assume they're too far off about whatever random things they dislike

Anonymous

Anonymous Fri 05 Mar 2021 16:10:52 No.12793013 Report

Quoted By:

if a is a integer and x is a prime and x is never greater than y and y is not divisible by x. use bezout's theorem to prove there exists a integer b such that for every integer o: x|a + yox-aby

Anonymous

Anonymous Fri 05 Mar 2021 16:40:48 No.12793135 Report

Quoted By: >>12793156

If you combine the following

$ -1x^-2 * e^x + x^-1 * e^x $
Is the result e^x(x^-3)? If not, how else do you sum it up?

Anonymous

Anonymous Fri 05 Mar 2021 16:45:40 No.12793156 Report

Quoted By: >>12793169

>>12793135
Wait it messed up
-x^-2 * e^x + x^-1 * e^x

Anonymous

Anonymous Fri 05 Mar 2021 16:48:00 No.12793165 Report

Quoted By:

Applied Differential Equations exam today, it's really easy but one thing I've been neglecting is how to graph and classify stuff properly. Here is a prime example:
>https://www.slader.com/textbook/9780321796981-differential-equations-and-boundary-value-problems-computing-and-modeling-5th-edition/346/problems/3/#

I can get the solution just fine, but I don't understand where this user got the equations to graph this (scroll down to just above the image).

Anonymous

Anonymous Fri 05 Mar 2021 16:49:26 No.12793169 Report

Quoted By: >>12793231

>>12793156
>>12793156
$-\frac{1}{x^2} e^x + \frac{1}{x} e^x = \frac{-e^x + xe^x}{x^2} = \frac{e^x(x-1)}{x^2} = e^x\left(\frac{1}{x}-\frac{1}{x^2}\right)$

Anonymous

Anonymous Fri 05 Mar 2021 17:02:45 No.12793231 Report

Quoted By: >>12793258 >>12793262

>>12793169
How you get to the second step is a bit confusing for me, probably due to lacking fundamentals. Like the x^2 staying at the bottom while the other x goes to the top. Can you break it down how you add those two together exactly?

Also I understand that using 1/x instead of x^-1 is maybe a bit faster, but it's possible to reach the result with that way of writing as well, right? Where did I go wrong?

Anonymous

Anonymous Fri 05 Mar 2021 17:09:12 No.12793258 Report

Quoted By: >>12793262 >>12793296

>>12793231
To sum fractions you need to have a common denominator, therefore you multiply the left one by $\frac{x}{x}$ to achieve that result.

> it's possible to reach the result with that way of writing as well, right?
$-x^{-2}e^x + x^{-1}e^x = x^{-2}(-e^x + xe^x) = x^{-2}e^x (x-1) = e^x(x^{-1} - x^{-2})$

Anonymous

Anonymous Fri 05 Mar 2021 17:10:15 No.12793262 Report

Quoted By: >>12793296

>>12793231
>>12793258
*you multiply the right one

Anonymous

Anonymous Fri 05 Mar 2021 17:14:06 No.12793275 Report

Quoted By:

Is there a version of the mean value theorem which holds for cauchy principal value integrals? What I mean is, can I say that
$pv \int_a^b f(x) g(x) dx = g(c) \bigg[ pv \int_a^b f(x) dx \bigg],$
for some $c$ if the function f has only one singularity within [a, b] and otherwise satisfies the MVT assumptions on both sides of the singularity. You can assume that I'm a brainlet

Anonymous

Anonymous Fri 05 Mar 2021 17:19:24 No.12793296 Report

Quoted By: >>12793303

>>12793258
>>12793262
Thank you. And to get to the last step? I understand -1/x^2 but why does x/x^2 result in 1/x?

Anonymous

Anonymous Fri 05 Mar 2021 17:21:12 No.12793303 Report

Quoted By:

>>12793296
$\frac{x}{x^2} = \frac{x}{x}\frac{1}{x} = \frac{1}{x}$

Anonymous

Anonymous Fri 05 Mar 2021 18:44:21 No.12793623 Report

Quoted By: >>12793661

do you normally have to be in the honors program to do undergrad research?

not sure if i should apply for honors, always thought it seemed kinda gay

Anonymous

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Anonymous Fri 05 Mar 2021 18:46:41 No.12793636 Report

Quoted By:

Is it normal to think about dropping out of graduate school every day?
Not really sure what I'm looking for in writing this post. I'm towards the end of the second year of my Ph. D. program in a STEM field. I have felt like I'm really not doing anything productive since COVID shutdowns started. I was only out of the lab for about 2 months. Current COVID guidance makes me uneasy about going in - do I really need to be there right now? The postdoc who I work with has told me a few times when I've been in working with him that he would perform an experiment and I don't need to stick around. Due to the nature of the work we do it isn't really possible to perform more experiments even if I were there, though I could help with some cleaning or preparation. Is doing clerical work such as making equipment/reagent purchases, and working on my reading / written exam documents, good enough? I feel like I am hardly contributing anything, which is giving me doubt as to whether I should continue in this program. At our weekly meetings it seems everyone else in my research group has some update on some experiments they are doing. Between getting my B.S. degree and entering the Ph. D. program I had an industry job where I knew where I fit in to the company, and could easily see how my work was useful. I feel as though I am more or less a leech here. The department has documents saying to consult with them as soon as possible if you are considering withdrawing from the program, but I don't have an exit strategy... I'm worried that starting that discussion will remove my option to stay in the program.

Thanks for reading my blog post. Also fuck reddit r/gradschool, they autodeleted this shit probably because I don't have enough reddit circlejerk points.

Anonymous

Anonymous Fri 05 Mar 2021 18:51:23 No.12793661 Report

Quoted By:

>>12793623
To do undergraduate research, you just need to reach out to your professors. Honors stuff isn't necessary, but it can help if the prof doesn't already know you since it shows that you're not a complete dumbass.

Anonymous

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Anonymous Fri 05 Mar 2021 19:28:44 No.12793811 Report

Quoted By: >>12794014

Why does this summation don't show up in the final answer?

Anonymous

Anonymous Fri 05 Mar 2021 19:51:05 No.12793925 Report

Quoted By: >>12793980

I worked through Hefferon's "Linear Algebra" a few weeks back. Would it make sense to go through Axler's "Linear Algebra Done Right" as well or does it have mostly the same material?
If it is very similar, what book would you recommend to read to get more into linear algebra?

Anonymous

Anonymous Fri 05 Mar 2021 20:01:08 No.12793980 Report

Quoted By: >>12794670

>>12793925
im not familiar with Hefferon's book enough to be sure, but LADR is intended as a second course on linear algebra, is more conceptual and proof-based not mechanical or computational like most first courses.

Anonymous

Anonymous Fri 05 Mar 2021 20:07:25 No.12794014 Report

Quoted By: >>12794198

>>12793811
The summations disappear because they're just considering each term of the sum individually - two of the integrals (2 & 4) go to zero for all n, while the first and third integrals goes to pi * the nth coefficient of the cosine series of f, and so their difference is 0 for all n.

Anonymous

Anonymous Fri 05 Mar 2021 20:24:21 No.12794096 Report

Quoted By:

>>12780620
It is a tensor, other guy is wrong. Equation is much more elegant in tensor notation but it will cost you a day to learn it

Anonymous

Anonymous Fri 05 Mar 2021 20:44:22 No.12794198 Report

Quoted By:

>>12794014
Thank you, anon.

Anonymous

Anonymous Fri 05 Mar 2021 21:05:28 No.12794317 Report

Quoted By: >>12794362 >>12794399 >>12794430 >>12794469 >>12794470

how do i solve something like this?
$ x - 3 y + 2 x y = 0, 4 x - 6 y - x y = 0] $

Anonymous

Anonymous Fri 05 Mar 2021 21:14:17 No.12794362 Report

Quoted By:

>>12794317
(0,0) is an obvious solution
For the other one, Gaussian elimination.

Anonymous

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Anonymous Fri 05 Mar 2021 21:20:42 No.12794399 Report

Quoted By: >>12794430

>>12794317
xy = 4x-6y
x-3y+2(4x-6y)=0 --> 9x = -15y --> x = -5y/3
4(-5y/3) - 6y - (-5y/3)*y = 0 --> 5(y^2)/3 = -38y/3 --> y^2 + 38y/5 = 0 --> y(y+38/5) = 0
so either y = 0, or y = -38/5
then x = -5y/3 --> x = -5*(38/5)/3 = -38/3
possible I made an arithmetic error in there so double check it for yourself.

Anonymous

Anonymous Fri 05 Mar 2021 21:26:01 No.12794430 Report

Quoted By: >>12794489 >>12794506

>>12794317
same as any system of linear equations: solve one equation for y in terms of x (or the opposite) then plug it in the other and solve it.
btw >>12794399's solution is wrong

Anonymous

Anonymous Fri 05 Mar 2021 21:33:10 No.12794469 Report

Quoted By: >>12794473

>>12794317
Multiply the first equation by $2$ for $2x - 6y + 4xy = 0$ . Subtract the second from it for $-2x + 3xy = 0$ .
Then $y = 3$ and you can solve for x.

Anonymous

Anonymous Fri 05 Mar 2021 21:33:15 No.12794470 Report

Quoted By: >>12794506 >>12794527

>>12794317
x-3y+2xy=0
x(1+2y) = 3y
x = 3y/(1+2y)

4x-6y-xy = 12y/(1+2y)-6y-3y^2/(1+2y)
= 12y/(1+2y)-6y(1+2y)/(1+2y)-3y^2/(1+2y) = 0
-> 12y-6y-12y^2-3y^2 = -15y^2+6y = 0
-> y = 6/15 = 2/5

x-3*(2/5)+2x*(2/5) = x -6/5 + 4x/5 = 9x/5 - 6/5 = 0
-> x = 2/3

Check again:
2/3 - 3*(2/5) + 2*(2/5)*(2/3) = 2/3 - 6/5 + 8/15 = 10/15 -18/15 + 8/15 =0

4*2/3-6*2/5-(2/5)*(2/3) = 8/3-12/5-4/15 = 40/15 - 36/15 - 4/15 = 0

Good thing I checked, because I made a mistake earlier. Always check your solution.

Anonymous

Anonymous Fri 05 Mar 2021 21:34:12 No.12794473 Report

Quoted By:

>>12794469
*for $-2x + 5xy$
* $y = 5$

Anonymous

Anonymous Fri 05 Mar 2021 21:37:02 No.12794489 Report

Quoted By: >>12794514 >>12794527

>>12794430
huh, my calculator is unable to do it, so i thought somehting funny must be going on.

so i have
$ y = \frac{-x}{-3x+2x} x = \frac{3y}{2y+1} $

and substituting for say x, then my original equation is
$ 3 y / (2 y + 1) - 3 y + 6 y^2 / (2 y + 1) = 0, 12 y / (2 y + 1) - 6 y - 3 y^2 / (2 y + 1) = 0 $

of which my calc still says no solution

Anonymous

Anonymous Fri 05 Mar 2021 21:41:07 No.12794506 Report

Quoted By:

>>12794470
>>12794430
thanks, I was trying to solve it like >>12794470. why is my calculator unable to do this?it handels normal systems fine

Anonymous

Anonymous Fri 05 Mar 2021 21:43:04 No.12794514 Report

Quoted By: >>12794527

>>12794489
what's the difference between normal systems and the one you input in your calculator?

Anonymous

Anonymous Fri 05 Mar 2021 21:45:53 No.12794527 Report

Quoted By: >>12794556

>>12794514
is this rhetorical, because I'm wondering the same. normal system usually don't have x*y, but that should be superficial.

even when I step through it by letting it solve for x or y (which does correctly as confirmed here: >>12794470, then I let solved variable take the new value, and try to solve for the remaining one, but it still says no solution.

this is output from my calculator: >>12794489

it normally solves these system fine, but something is diffrent here and Im not sure what

Anonymous

Anonymous Fri 05 Mar 2021 21:52:18 No.12794556 Report

Quoted By: >>12794614

>>12794527
>is this rhetorical
yes
calculator solvers are pretty dumb, you need to input simple equations. your equation has rational fractions which can fuck with your calculator as it attempts to solve it numerically and encounters a singularity. try to get rid of the (2y+1) denominators, it should make stuff neater

Anonymous

Anonymous Fri 05 Mar 2021 22:05:08 No.12794614 Report

Quoted By:

>>12794556
huh, weird. usually it's pretty good. thanks for the help. unfortunately, getting rid of the denominators didn't fix it. fwiw im not worried about the algebra, just about how to get my calculator to solve these sorts of things so i can check my work and go fast on exams.

Anonymous

Anonymous Fri 05 Mar 2021 22:16:21 No.12794670 Report

Quoted By: >>12794692 >>12794699

>>12793980
Thanks! I thought about it some more and might just pirate it to get a feel for how similar they are.

Anonymous

Anonymous Fri 05 Mar 2021 22:21:30 No.12794692 Report

Quoted By:

>>12794670
i didn't finish LADR but i really liked the parts I did, it was at just the right level of rigor for me. I like the typesetting and style in the second edition more as well.

Anonymous

Anonymous Fri 05 Mar 2021 22:22:32 No.12794699 Report

Quoted By:

>>12794670
oh, Axler also has lectures on YT for the book

Anonymous

Anonymous Sat 06 Mar 2021 01:17:42 No.12795286 Report

Quoted By: >>12796209

How do you know if a quadrangle is a square in a certain perspective?

Anonymous

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Anonymous Sat 06 Mar 2021 02:23:45 No.12795474 Report

Quoted By: >>12795577 >>12796216

Linux Question sort command

I have to sort through lines but I want to ignore number.

My files are in this order 'numbers;numbers;Text'

I've tried with-k but without luck.

Anonymous

Anonymous Sat 06 Mar 2021 02:56:26 No.12795577 Report

Quoted By: >>12795614

>>12795474
Does ; mean a newline or a csv-like file or something else?

Either way you can't do it with just sort.

Anonymous

Anonymous Sat 06 Mar 2021 03:07:25 No.12795614 Report

Quoted By: >>12795624

>>12795577
Don't think It means new line bc I tried -k 3.3

They are simply separated by ;
No space or anything else.

I have 10 lines like it (#;#;text)
so it's not possible?

While we are at it, can I excuse tr replacements simultaneously?
Like tr 'a' 'b' and 'c' 'd'

I tried to put add (), {}, && between them but with no success

Anonymous

Anonymous Sat 06 Mar 2021 03:09:47 No.12795624 Report

Quoted By: >>12795651

>>12795614
if its always in the format #;#;text then

cut -d';' -f3 file.txt | sort

Anonymous

Anonymous Sat 06 Mar 2021 03:14:17 No.12795651 Report

Quoted By: >>12795667

>>12795624
I can only use grep and sort, I guess I'm fucked.
I don't think grep can send back while ignoring numbers.

Anonymous

Anonymous Sat 06 Mar 2021 03:16:52 No.12795667 Report

Quoted By: >>12795676

>>12795651
why da fuck can you only use grep and sort?

Anonymous

Anonymous Sat 06 Mar 2021 03:18:52 No.12795676 Report

Quoted By: >>12795689

>>12795667
I'm not working on my own shelf, it's my professor whos forcing us to do this shit as a test.

Anonymous

Anonymous Sat 06 Mar 2021 03:21:19 No.12795689 Report

Quoted By: >>12795711

>>12795676
egrep -E -e'[a-zA-Z]+$' -o file.txt | sort

Anonymous

Anonymous Sat 06 Mar 2021 03:25:05 No.12795711 Report

Quoted By:

>>12795689
I'll try to work with this, Cheerio mate

Anonymous

Anonymous Sat 06 Mar 2021 06:13:06 No.12796209 Report

Quoted By: >>12796223

>>12795286
Any convex quadrilateral can be represented as a projective transformation of a square. Given the four vertices of the unit square [u_i,v_i] = [±1,±1] and the four vertices of the quadrilateral [x_i,y_i], transformation by a 3x3 matrix followed by projective division gives you eight equations:
x_i = (c11.u_i+c12.v_i+c13)/(c31.u_i+c32.v_i+c33)
y_i = (c21.u_i+c22.v_i+c23)/(c31.u_i+c32.v_i+c33)
Add a ninth equation c33=1 to get nine equations in nine unknowns (c_ij) which can then be solved.

The math actually works for a non-convex quadrilateral, but you'll end up with an inversion (points inside the square map to points outside the quadrilateral, including the point at infinity).

Anonymous

Anonymous Sat 06 Mar 2021 06:17:55 No.12796216 Report

Quoted By: >>12796305

>>12795474
Did you set the field separator with -t?

sort -t\; -k3 ... should work (you need to quote or escape the semicolon if using the shell).

Anonymous

Anonymous Sat 06 Mar 2021 06:20:59 No.12796223 Report

Quoted By:

>>12796209
How do you know it's not a rectangle or a rhombus?

Anonymous

Anonymous Sat 06 Mar 2021 06:54:49 No.12796305 Report

Quoted By:

>>12796216
Yeah it worked, thanks. What does the '\' mean? It erases the semicolon?

btw Im trying to do many letter replacement at the same time with tr. Any idea how can I do it at once instead of piping and using tr every time?

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