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Math Help - Basic representation question...

  1. #1
    Junior Member mant1s's Avatar
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    Basic representation question...

    Hi guys,

    What is another way of representing x sin (x). I forgot the identity?

    Code:
    Let f(x) = x sin (x)
    
    find f''(2)
    Real simple I know but I just forgot the other way of representing x sin (x)

    Thanks!
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  2. #2
    Senior Member apcalculus's Avatar
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    Quote Originally Posted by mant1s View Post
    Hi guys,

    What is another way of representing x sin (x). I forgot the identity?

    Code:
    Let f(x) = x sin (x)
    
    find f''(2)
    Real simple I know but I just forgot the other way of representing x sin (x)

    Thanks!
    Why don't you just differentiate twice?

    f'(x) = sin x + x cos x
    f"(x) = cos x + cos x - x sin x = 2 cos x - x sinx

    f"(2) = 2 cos 2 - 2 sin 2

    I might be missing something about 'representations' here but it looks pretty straight-forward twice differentiation task.
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  3. #3
    Junior Member mant1s's Avatar
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    Quote Originally Posted by apcalculus View Post
    f'(x) = sin x + x cos x
    Thanks for your help, that's what I was getting confused on.

    cheers,

    -M
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