A community for students.

Here's the question you clicked on:

55 members online
  • 0 replying
  • 0 viewing

anonymous

  • 5 years ago

I am having a problem with a this problem: Given the function: f(x) = 8(cos(x))^2 - 16sin(x) where 0 <= x <= 2pi Find the intervals where f is increasing and decreasing, find the local min and max and find the inflection points

  • This Question is Closed
  1. Yuki
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    To find where f is increasing or decreasing, first you have to find the slope of f, " f' " right? Were you able to find the derivative of f ?

  2. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    Yes, the derivative of f is -16cos(x)(-sinx)- 16cosx

  3. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    At least thats what I think

  4. Yuki
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    Good. Now what we need to do is to find where the slope is increasing. In other words, where f' > 0. So we will have to solve for 16cos(x)sin(x)-16cos(x) >0 would you be able to do this ?

  5. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    need help

  6. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    I'm having a problem doing this ... my brain is fried

  7. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    um, i think the derivative is maybe : 16cos(x)(-sin(x)-16cos(x) ...

  8. Yuki
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    Ok, when you are solving for trig then these are the usual steps that you are going to take. if you can see that 16cos(x) is common in both terms, then you should factor it. it will look like 16cos(x) {1-sin(x)} >0 . Do you know what to do from here ?

  9. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    We can factor -16cosx, so -16cos(x)(sinx +1)

  10. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    That means f'x = 0 when sinx=-1 or cos(x)=0, right?

  11. Yuki
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    Yes, but since this is an interval you have to figure out where f' is positive or negative. before we proceed, can you find x ?

  12. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    The problem I'm having is the intervals on the X-axis ... would we set them up at the critical numbers and the endpoints? cos(x)=0 at pi/2 and 3pi/2 sin(x) = -1 then x = 3pi/2

  13. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    not getting x

  14. Yuki
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    Ok, so you understand that those x's are the points where f' = 0. Now to check the interval, all you have to do is to plot those x's on the numberline and check the intervals between them. In your case the intervals you will check is [0,pi/2), (pi/2, 3pi/2), (3pi/2,2pi]. so for example, if I want to check whether f' is positive or negative on the first interval, you just plug in a convenient number in it, such as pi/4 or pi/6. if f' >0, it will be true for all x's in that interval.

  15. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    mean value theorem?

  16. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    I think I've got it

  17. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    72500-60900=11600*7=81200 u guys r awesome

  18. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    catch guys nex time Thnks again

  19. Yuki
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    hahaha. No no you don't have to do anything like that. to check the first interval [0,pi/2) you just plug in a convenient value x into f'. If I plugged in x = pi/4, I can see that 16cos(pi/4) {1-sin(pi/4)} is negative since 16(sqrt2)/2 >0 and 1- sqrt(2)/2 is negative. so I know that on the interval [0, pi/2), f' <0 therefore f is decreasing.

  20. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    And if you plug in, say 5pi/4? it's increasing on the second interval and decreasing on the third

  21. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    What about the inflection points?

  22. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    I guess it's where f'(x) changes sign?

  23. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    Or do i need to get f''(x)

  24. Yuki
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    To find the inflection points you will find f" and let it equal to 0. Then you will do the same thing again to determine if it really is an "inflection point" If f" = 0, that doesn't necessarily mean that it is an inflection point right? so you will check the interval again to see if the concavity "changed" meaning f" >0 becomes f" <0 before and after those points where f" =0. It might be an overkill for this problem, but if you want to be 100% sure then you should check it. to find out whether f has a maximum or a minimum at those x's where you had f' =0, you will plug those x's into the f". if f">0, f is concave up meaning that at that point the graph looks like a " U " so we know it will be a minimum. if f"<0 it becomes a maximum.

  25. Yuki
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    I need to go, but if you need more help, ask others about how to find an inflection point and how to use the "second derivative test." They should be able to help you out :) It was fun talking to you. Yuki

  26. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    I don't know if I can do anymore and my homework is due. I just can't think. My brain is friend from 2 big software projects

  27. Not the answer you are looking for?
    Search for more explanations.

    • Attachments:

Ask your own question

Sign Up
Find more explanations on OpenStudy
Privacy Policy

Your question is ready. Sign up for free to start getting answers.

spraguer (Moderator)
5 → View Detailed Profile

is replying to Can someone tell me what button the professor is hitting...

23

  • Teamwork 19 Teammate
  • Problem Solving 19 Hero
  • You have blocked this person.
  • ✔ You're a fan Checking fan status...

Thanks for being so helpful in mathematics. If you are getting quality help, make sure you spread the word about OpenStudy.

This is the testimonial you wrote.
You haven't written a testimonial for Owlfred.