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Math Help - Help with a Trig Proof

  1. #1
    Newbie
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    Sep 2008
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    Help with a Trig Proof

    The question says:

    Given y = xsinx, prove that y" + y' = 2cosx.

    I found that y' = sinx + xcosx, and y" = 2cosx - xsinx.

    But I cannot figure out how to simplify (2cosx - xsinx) + (sinx + xcosx) so that it equals 2cosx.

    Help?

    Jeannine
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  2. #2
    Super Member flyingsquirrel's Avatar
    Joined
    Apr 2008
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    Hi,
    Quote Originally Posted by jbecker007 View Post
    The question says:

    Given y = xsinx, prove that y" + y' = 2cosx.

    I found that y' = sinx + xcosx, and y" = 2cosx - xsinx.
    That's correct.
    But I cannot figure out how to simplify (2cosx - xsinx) + (sinx + xcosx) so that it equals 2cosx.
    I think the question should be "Given y = x\sin x, prove that y'' + y = 2\cos x".
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