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Posted by on Tuesday, July 22, 2008 at 7:35am.

Find the general solution of the following equation.
sin (5x) = cos (3x)
sin (6x) sin (x) = cos (3x) cos (4x)

Thanks for your help :)

  • Trigonometry - , Tuesday, July 22, 2008 at 7:42am

    Those are two different equations.
    Which one do you want to solve?

  • Trigonometry - , Tuesday, July 22, 2008 at 7:47am

    I've tried the first one, and i found out that x=45n+11.25 or x=45-180n but i don't know if i am correct or not.
    The second one I just cant do it.

  • Trigonometry - , Tuesday, July 22, 2008 at 10:38am

    No one helped me ?!
    How poor I am ..........

  • Trigonometry - , Tuesday, July 22, 2008 at 11:02am

    I could write both equations in terms of sines and cosines of x, but they become an incredible mess. There may be an elegant solution to either, but don't see it.

  • Trigonometry (shortcut) - , Tuesday, July 22, 2008 at 3:10pm

    sin (5x) = cos (3x)
    That relationship is true if the argument of the sin term is y and the argument of the cos term is pi/2 - y.
    Let
    5x = y, and
    3x = pi/2 -y
    Now solve for y.
    15x = 3y
    15x = 5 pi/2 -5y

    0 = 8y - 5 pi/2
    y = 5 pi/16 = 56.25 degrees is an answer

    There may be another answer as well, since the
    sin a = cos b
    relationsip is also true whenever the argument of the sin term is y and the argument of the cos term is 3 pi/2 + y.

    So let

    5x = y
    3x = 3 pi/2 + y
    15x = 3y
    15x = 15 pi/2 + 5 y
    2y = -15 pi/2
    So y = -15 pi/4 = -675 degrees, or -315 degrees, or +45 degrees is another answer.

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