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Math Help - Trigonometric Derivative using Identity

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    Trigonometric Derivative using Identity

    Use the identity cos 2x = cosx * cos x - sin x * sin x to find the derivative of cos 2x. Express the derivative in terms of sin 2x.

    I have no idea how to do this.
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    is up to his old tricks again! Jhevon's Avatar
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    Quote Originally Posted by Truthbetold View Post
    Use the identity cos 2x = cosx * cos x - sin x * sin x to find the derivative of cos 2x. Express the derivative in terms of sin 2x.

    I have no idea how to do this.
    just let y = \cos x \cos x - \sin x \sin x and use the product rule
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    Quote Originally Posted by Jhevon View Post
    just let y = \cos x \cos x - \sin x \sin x and use the product rule
    Product rule of both sides?

    Get: -cos sin + cos sin - (-cos sin - cos sin)
    And get: 0

    That cannot be the right answer.

    What am I doing wrong?
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    is up to his old tricks again! Jhevon's Avatar
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    Quote Originally Posted by Truthbetold View Post
    Product rule of both sides?

    Get: -cos sin + cos sin - (-cos sin - cos sin)
    And get: 0

    That cannot be the right answer.

    What am I doing wrong?
    the derivative of sin(x) is +cos(x)

    and it's a bad habit to write sin and cos alone like that.
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