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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Originally Posted by Truthbetold 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 $\displaystyle y = \cos x \cos x - \sin x \sin x$ and use the product rule
Originally Posted by Jhevon just let $\displaystyle y = \cos x \cos x - \sin x \sin x$ 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?
Originally Posted by Truthbetold 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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