Answer
$$\frac{dy}{dx}=3\cos(\tan3x)\sec^23x$$
Work Step by Step
$$\frac{dy}{dx}=(\sin(\tan 3x))'=\cos(\tan3x)\cdot(\tan3x)'=\cos(\tan3x)\sec^23x\cdot(3x)'=\cos(\tan3x)\sec^23x\cdot3=3\cos(\tan3x)\sec^23x$$
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