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Homework Help: Find a,b,c in sin^5(x) = asin(x) + bsin(3x) + csin(5x)

  1. Nov 1, 2009 #1
    1. The problem statement, all variables and given/known data
    Find A,B,C in sin^5(x) = Asin(x) + Bsin(3x) + Csin(5x).

    3. The attempt at a solution

    I get by Euler the double angle identities for [itex]sin(3x) and sin(5x)[/itex].
    They are

    [tex] sin(3x) = s(2x) c(x) + c(2x) s(x) [/tex]
    [tex] sin(5x) = s(3x) c(2x) + c(3x) s(2x) [/tex]

    I have the following expression now


    sin^5(x) = 1 - cos^5(x) - 5c^4(x)s(x) - 10c^3(x)s^2(x) - 10 c^2(x)s^3(x) - 5 c(x)s^4(x) [/tex]

    where the trigonometric terms are [itex] (1/2) s(2x) c^3(x), (1/4) (s(2x))^2 c(x), (1/4) (s2x)^2 s(x), (1/2) s(2x) s^3(x) [/itex], respectively.

    I get the common term [itex] [(1/2)s(2x) = c(x)s(x) [/itex] by the double angle identity of sine.

    So you have [itex] (-5/2) s(2x) (c^3(x) + c(x) + s(x) + s^(x) [/itex].

    By comparing the terms to the given equation, we get

    [tex] Bsin(3x) = B (s(2x) c(x) + c(2x) s(x) ) [/tex]
    [tex] Csin(5x) = C ( s(3x) c(2x) + c(3x) s(2x) ) [/tex]

    so we have

    [tex] B c(x) + C c(3x) = (-5/2) ( c^3(x) + c(x) + s(x) + s^3(x) ) [/tex]
    which implies that [itex] C = - \frac {-5} {2} [/itex]

    The other term is in the form

    [tex] B c(x) = (-5/2) ( c(x) + s(x) + s^3(x) ) [/tex]
    where where [itex] s(x) + s^3(x) = s(x) (1 + s^(x) = 2s(x) - c^2(x) s(x) [/itex] by Pythagoras.

    However, I do not see directly how to get the term [itex] c(x) [/tex] to the RHS of the equation.


    My first attempt seems to be useless, since the answer may be found by Fourier series too.
    However, I have little experience of them and cannot see to how use them here.

    How can you find A, B and C by Fourier series or by other method?
  2. jcsd
  3. Nov 10, 2009 #2


    User Avatar
    Science Advisor
    Homework Helper

    Hi soopo! :smile:

    How about expanding (eix - e-ix)5 ? :wink:
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