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Mathematics 12 Online
OpenStudy (anonymous):

Why does the derivative of f(x)=(5cos^2 8x) equal f'(x)=-40sin(16x) and not f'(x)=-80cos(8x)sin(8x) ?

zepdrix (zepdrix):

Both answers are correct :) The first one is because they (the book or wherever you're getting your answer from) has simplified the answer by applying the double angle formula for the sine function! :D

OpenStudy (anonymous):

Ohhh! Yeah, I was confused by it. haha. Wait, what is the double angle formula? I don't remember it:(

zepdrix (zepdrix):

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