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Math Help - 9 sin(2α) sin(3α)

  1. #1
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    9 sin(2α) sin(3α)

    This math problem has been giving me a headache. I understand the product-to-sum formula to be: sin u * sin v = (1/2)[cos(u-v)-cos(u+v)], but when I work out the problem I get: (9/2)(-cos(a) - cos(5a)). Is that wrong?
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  2. #2
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    Quote Originally Posted by sparker2000 View Post
    This math problem has been giving me a headache. I understand the product-to-sum formula to be: sin u * sin v = (1/2)[cos(u-v)-cos(u+v)], but when I work out the problem I get: (9/2)(-cos(a) - cos(5a)). Is that wrong?
    correct solution but

    \cos (-a) = \cos a

    so the answer

    \frac{9}{2} \left( \cos (2a-3a) - \cos (2a+3a) \right)

    \frac{9}{2} \left( \cos (-a) - \cos (5a) \right)

    \frac{9}{2} \left( \cos (a) - \cos (5a) \right)
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