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# How do you find the derivative of y=cos 3x?

##### 1 Answer
Feb 7, 2015

You can use the Chain Rule: first you derive the $\cos$ as it is (without changing its argument) and then multiply by the derivative of the argument, $3 x$.

So you get:

$y ' = - \sin \left(3 x\right) \cdot 3 = - 3 \sin \left(3 x\right)$