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*Seond Order Derivative*

  1. Jun 13, 2008 #1
    Been doing calculus all my life , that's why i am at the Indian Institute of Technology:shy:..
    but never faced a headache like this......

    Prove that:

    If x=cos theta and y = sin^3 theta

    then [d^2y/dx^2] + [dy/dx]^2 = 3sin^2 theta [ 5 cos ^2 theta -1 ]

    My side:

    dx/d theta = -sin theta
    dy/d theta = 3 sin^2 theta cos theta
    >> {dy/dx}== -3 sin theta cos theta

    now d^2y/dx^2= -3 [ cos^2 theta - sin ^2 theta ]* 1/-sin theta

    moving with the give expression....

    3cos^2 theta-3 sin^2 theta/sin theta + 9 sin^2 theta cos^2 theta

    Spent a lot of time...cud not get anywhere...:grumpy:
    Am i messing up the trigo functions?:frown:

    This is not a homework question. I don't have to do homework anymore.I work in a MNC.
    Last edited: Jun 13, 2008
  2. jcsd
  3. Jun 13, 2008 #2

    Ben Niehoff

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    Science Advisor
    Gold Member

    You've never faced a "headache" like that? Use trig identities.
  4. Jun 14, 2008 #3
    Please be a bit more clear.Even a moron knows that you gotta use trigo identities.Its no rocket science!!!!
  5. Jun 14, 2008 #4
    Ok, I'm going to assume you didn't make any algebra/differentiation mistakes so far (I didn't check)
    If that is the case, then you are starting here:
    3cos^2 theta-3 sin^2 theta/sin theta + 9 sin^2 theta cos^2 theta

    And you want to get here:
    3sin^2 theta [ 5 cos ^2 theta -1 ]

    I would write each expression in terms of complex exponentials, then multiply all the exponentials out completely, working both backwards and forwards simultaneously. Then when your results meet in the middle, rearrange your steps to go from where you started to where you finished.

    Here is a small (easier) example of this strategy in action: Say we want to show [itex]cos^2\left(x\right) = 1-sin^2\left(x\right)[/tex]
    Working forwards:
    [tex]cos^2\left(x\right)=\left(\frac{e^{i x}+e^{-i x}}{2}\right)^2=\frac{e^{2 i x} + 2 + e^{-2 i x}}{4}[/tex]
    Now hold that thought while we work backwards...

    Working backwards:
    [tex]1-sin^2\left(x\right) = 1-\left(\frac{e^{i x}-e^{-i x}}{2 i}\right)^2=1-\frac{e^{2 i x}-2+e^{-2 i x}}{-4}=\frac{e^{2 i x} + 2 + e^{-2 i x}}{4}[/tex]

    Then to write the final proof, you would just rearrange the steps:
    [tex]cos^2\left(x\right)=\left(\frac{e^{i x}+e^{-i x}}{2}\right)^2=\frac{e^{2 i x} + 2 + e^{-2 i x}}{4}=1-\frac{e^{2 i x}-2+e^{-2 i x}}{-4}=1-\left(\frac{e^{i x}-e^{-i x}}{2 i}\right)^2=1-sin^2\left(x\right)[/tex]
    Last edited: Jun 14, 2008
  6. Jun 16, 2008 #5
    Dear maze, thanks a lot for your efforts and spending your precious time on the problem.
    Your method is really impressive and shows your multidimensional thinking.But the problem is that the method you supplied is a lengthy one and can be extremely confusing in long trigo expressions like mine is the case.I would like a simple approach.
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