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Math Help - problem find the Exact Value of cos35 sin55 + sin 35 cos 55?????

  1. #1
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    problem find the Exact Value of cos35 sin55 + sin 35 cos 55?????

    i am having trouble remember how to solve this problem without a calculator any words of wisdom would be greatly appreciated
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  2. #2
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    Quote Originally Posted by Gamaltes View Post
    i am having trouble remember how to solve this problem without a calculator any words of wisdom would be greatly appreciated
    you need this identity ...

    \sin(x+y) = \sin{x}\cos{y}+\cos{x}\sin{y}
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    but how do i solve from there... sorry I'm really lost
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    Quote Originally Posted by Gamaltes View Post
    i am having trouble remember how to solve this problem without a calculator any words of wisdom would be greatly appreciated
    The answer ought to be 1, since

    35^o+55^o=90^o

    Therefore, viewed from within a right-angled triangle

    cos35^o=sin55^o

    cos55^o=sin35^o

    so

    cos35^osin55^o+sin35^ocos55^o=cos^235^o+sin^235^o=  sin^255^o+cos^255^0=1
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  5. #5
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    AAAAAAHHHH there it is.... now i remember.... thanks a lot folks
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