# Calcus homework help because I'm too dumb to help my kid, please.

Discussion in 'General Discussion' started by digitalwanderer, Oct 8, 2019.

1. ### digitalwanderer Dangerously Mirthful Legend

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She had some problems with her first calculus teacher about some things that the professor turned out to be VERY wrong about so she switched classes, is filing a complaint, and the department head is looking in to the matter. Purdue has been really good about it or I'd be raising holy hell, but they really do take care of their students.

Any help is appreciated.

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#1
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2. ### pcchen Moderator ModeratorVeteranSubscriber

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For #1:

Knowing that derivative of arctan(x) is 1/(1+x^2).

let t = arctan(2x) so y = t^2, dy/dx = dy/dt * dt/dx = 2t * (arctan(2x))'
let s = 2x so again (arctan(s))' = 1/(1+s^2) * 2
so dy/dx = 2*arctan(2x) * 1/(1+4x^2) * 2 = 4 * arctan(2x) / (1+4x^2)

For #2:

(a) by "implicit differentiation" it means doing y' without solving y first
so 9x^2-y^2=1 -> 2*9x - 2y * y'=0
therefore y' = 18x/2y = 9x/y

(b) solve y first:
y^2 = 9x^2 - 1
=> y = +/- sqrt(9x^2 - 1), or (9x^2-1)^0.5
therefore
y' = +/- 0.5 * (9x^2-1)^(-0.5) * (9x^2-1)' = +/- 0.5 * (9x^2 - 1)^(-0.5) * 18x = +/- 9x/sqrt(9x^2-1)

It can now be verified that y' is indeed 9x/y.

#2
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3. ### orangpelupa Elite Bug Hunter LegendVeteran

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Oh my poor brain

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4. ### digitalwanderer Dangerously Mirthful Legend

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Thank you pcchen!

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5. ### iroboto Daft Funk LegendRegularSubscriber

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thanks man, this brings back memories

#5

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thank you pcchen!! (im digi's daughter)

i also um. just wanted to ask and see if this looks okay.

am i doing these right? im struggling to be able to describe the true/false problems... especially because i dont feel confident in what im doing. the joys of transferring into a class that's way ahead of the one you were in...

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7. ### pcchen Moderator ModeratorVeteranSubscriber

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For #1 I think you are doing great.

#2 are all "prove or disprove this" questions and they can be quite annoying

For (a):
To explain it more rigorously, if f(x) is concave up for all x and f''(x) exists for all x, f''(x) > 0 for all x and vice versa. Therefore, if f(x) and g(x) are both concave up for all x, f''(x) > 0 and g''(x) > 0, so f''(x) + g''(x) > 0 for all x, therefore f(x)+g(x) is also concave up for all x.

For (b): your counter example is good but it's not about the order of multiplication though, it's just that both (fg)' and f'g' are zero.

For (c): it's actually not true because for example y = sin(x) + 100x also have y'''' = sin(x)

For (d): to prove it more rigorously, mathematical induction should be used: if y = a^x then y' = ln(a) * a^x, so if nth derivative is (ln(a))^n * a^x, (n+1)th derivative is (ln(a))^n * ln(a) * a^x = (ln(a))^(n+1) * a^x, QED

For #3 I think you missed a y' as (4y^3*y')' = 12y^2 * y' * y' + 4y^3 * y'', not 12y^2 * y' + 4y^3 * y''
so it's y'' = (-12x^2-12y^2(y')^2)/(4y^3) = (-3x^2-3y^2(-x^3/y^3)^2)/(y^3) = (-3x^2-3y^2x^6/y^6)/(y^3) = (-3x^2-3x^4/y^4)/(y^3) = -3x^2(y^4 + x^4)/y^7
and since x^4 + y^4 = 16, so it's -3x^2(16)/y^7 = -48x^2/y^7

I guess it's common to not feel confident enough when doing mathematical proof though, even professional mathematicians make mistake in papers

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8. ### orangpelupa Elite Bug Hunter LegendVeteran

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I leave this page open on my work computer when I leave it to got to the toilets or grab some food.

Really good for making it looks like I'm working on something serious and complicated and not to be disturbed.

(sorry, I can't help with anything, my brain instantly goes blank reading those numbers formula that looks as meaningful as hieroglyphs to me)

#8

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thank you. i really need the support and reassurance because... ow. and i'm pretty positive nobody at my house is able to help me because... yeah. calculus.

this really helped me out though!! i went to my professor's office hours (first time i've done that in college ) and since he just collected this he graded mine and i got a 48/50!! i went because another student asked him if they could do practice questions that modeled the ones that would be on our quiz today. between the two of us, i was the one who was understanding the concepts we were applying more

i think i did well on it... it was open note and book, though, AND i spent too much time trying to understand how to use a program i put on my calculator last night to find the volume of a sphere, BUT i found it after 15 minutes or so...

yea i hate these kind of questions

this was the one he took two points off of because i didn't show the entire process of getting to the fourth derivative. but that's... that's whatever to me. i still got an a. he just wanted a little bit more.

i think this is what genuinely helped the most. i had gone to my college's tutoring center for walk in tutoring to try and get this done (that's where i took the picture i posted) and the girl i talked to... i don't think she wasn't helpful, per say, but she's in higher level math courses and i could get forgetting what's taught in calc 1 when you've done multivariable calc. i've also just had a lot of trouble doing implicit differentiation and this helped me understand what the hell i'm supposed to be doing. although, i'm still getting used to reading and typing out math shit so i did have to google an ascii math converter to read this

it's probably because i don't have a strong understanding of the subject yet, but some of the things you have to do to equations and numbers in calc seem impossible. it feels like fake math but i'm working on it. it's cool as hell to me. i still can't believe i like math now but here we are

ANYWAYS. all that aside (can anyone tell that i'm my dad's kid by how much i write ) thank you for your help. i never thought i'd be able to do calculus, yet alone really be in college. i know a lot of my confusion can stem from me just... not knowing some of the basics. like when we were still just doing limits and we were dealing with delta epsilon problems?? i honestly still don't get those. but i don't know how often they'll really come up, so i'm... not too worried about it right now.

#9

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that's ok. i still sort of feel that way about it while i see them too. math is hard.

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11. ### pcchen Moderator ModeratorVeteranSubscriber

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Great to hear it helped!

If what you are going to do is mostly on the practical side, I'd say don't worry too much about the delta epsilon things These are more about making rigorous proof and definitions. They are very important for mathematicians, but not really that important for real world applications. (and yes, there are real world applications of calculus! ) It's more important to understand the general idea of derivatives and integral (e.g. derivatives are rates of changing of a function, and integrals are the area surrounded by the function, etc.)

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12. ### homerdog donator of the year LegendVeteranSubscriber

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Oh I took calculus 1 and 2 in college, the hard versions for science and engineering students. Lemme help.

Holy fuckin shit nevermind good luck.

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hi calculus is hell. i feel like im personally kind of falling behind since we move so fast and i havent had enough time to learn the concepts

anyways i have another worksheet that i hope im doing right but have NO clue if i am. if someone is willing to check id appreciate it. (and if you need better pics - feel free to let me know.)

i took a pic of the bottom part since its very... light. i tried making it funner with color but it still hurts my brain. how the hell do i actually absorb this information. what is going on. how do i even have a C+ in there.

i got a 5 question OPEN NOTE quiz back that i spent 2 hours on and i got an 18/25. genius brain.

also actually too on top of that- unrelated to math, but how people get enough sleep? i used to be able to, but since college started and i switched into an 8am class...

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14. ### iroboto Daft Funk LegendRegularSubscriber

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lol welcome to the club. I never really realized how bad I was at math and abstract concepts until university hit. My lowest grades were always the maths/physics. I've normally a B+ ~ A- student. I had to get used to seeing Ds and C-.

I would describe my entire university career to be like that. shit moves SOOOO fast. A single lecture missed is painful.
Just hang on, do the study group thing. Review the previous tests for your course in your library. Review your lecture notes every day for every course. A lot of this stuff, it's not easy to get right away. Takes time.

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15. ### pcchen Moderator ModeratorVeteranSubscriber

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I didn't check your calculation for #1 but I think your general idea is correct. Basically check where v' = 0 and also check the boundaries (t=0 and t=126).
I think you got them right for #2 and #3, well done!

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i get that... i'm dealing with that right now. the last two years i was in high school i was getting all As and i'd panic if i got a B+ on something. but the C+ in my calc class is okay? i don't know if it's because i'm still in such disbelief that i'm even in calculus AND i'm passing it... or that i'm in college (they really let me in??)... maybe i've matured, but i doubt that.

yeah. yeah. i missed the lecture for the first section of this chapter and it's made it a lot harder. and my calc lectures are the shortest ones i have- only 50 minutes. every day. at 8am. i haven't gotten enough sleep in the last month...

yay! thank you!
i understand not checking the actual calculations for number 1... ugly numbers... when i was doing them i was getting annoyed with my calculator (but that's more on me than it; it's still new to me, i still don't fully understand how to use it... )

i got to my lecture late today and gave him that worksheet when he gave us some time to work on an antiderivative problem (we just started... went and googled them to try and find a video for later to watch to sort of get it and i'm... ) and he was chuffed by how colorful it was. i forgot it's probably uncommon for a math class...

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17. ### homerdog donator of the year LegendVeteranSubscriber

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As previously mentioned I took calculus 1 twice and it wasn't because I liked it so much
To my surprise it was extremely easy the second time around and I got an A.

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18. ### digitalwanderer Dangerously Mirthful Legend

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Thank you all so much, this is WAAAY beyond my maths level!

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19. ### Sxotty LegendVeteran

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Hey, I have a question. When you are integrating a piecewise function, why can't you just do an indefinite integral and then apply the limits to the resulting equations after the fact? For example.

dx/dt=b*sin(a*t) (0<t<T) and dx/dt=c (t>T)
I thought I could just do x=-b/a*cos(a*t) and then plug in a value between 0 and T, but apparently this is wrong.

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20. ### pcchen Moderator ModeratorVeteranSubscriber

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I'm not sure what the problem is, but to me it looks like x = -b/a*cos(a*t) + C1 between 0 and T and x = ct + C2 for t > T.
If the function is continuous then -b/a*cos(a*T) + C1 = cT + C2 and that could give us some idea about the relation between C1 and C2.

#20
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