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Math Help - exact value of trig function

  1. #1
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    exact value of trig function

    What is the exact value of

    sin36
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  2. #2
    Math Engineering Student
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    \sin36^\circ=\sin(2\cdot18^\circ)=2\sin18^\circ\co  s18^\circ.

    \sin36^\circ=\cos54^\circ\implies2\sin18^\circ\cos  18^\circ=\cos(3\cdot18^\circ), so \cos(3\cdot18^\circ)=4\cos^318^\circ-3\cos18^\circ.

    Now, 2\sin18^\circ=4\cos^218^\circ-3.

    From here you can get \sin18^\circ & \cos18^\circ, then plug'em into the initial formula and you're done.
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