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Math Help - some trig functions

  1. #1
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    some trig functions

    i have some questions i really need help with

    sin3 A = 3sin A - 4sin^3 A
    (Hint use sin3 A = sin(2a + a)

    and

    sin3 x / sin x - cos3 x / cos x = 2

    please help im stuck
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  2. #2
    is up to his old tricks again! Jhevon's Avatar
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    Quote Originally Posted by dave30001 View Post
    i have some questions i really need help with

    sin3 A = 3sin A - 4sin^3 A
    (Hint use sin3 A = sin(2a + a)
    um, use the hint! did you try? what did you get?

    and

    sin3 x / sin x - cos3 x / cos x = 2

    please help im stuck
    I will start you off:

    Consider the LHS:

    \frac {\sin 3x}{\sin x} - \frac {\cos 3x}{\cos x} = \frac {\sin 3x}{\sin x} \cdot \frac {2 \cos x}{2 \cos x} - \frac {\cos 3x}{\cos x} \cdot \frac {2 \sin x}{2 \sin x}

    = \frac {2 \sin 3x \cos x}{2 \sin x \cos x} - \frac {2 \sin x \cos 3x}{2 \sin x \cos x}

    = \frac {2 \sin 3x \cos x - 2 \sin x \cos 3x}{\sin 2x}

    now can you finish up?
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