# Chapter 3: Derivatives - Section 3.6 - The Chain Rule - Exercises 3.6 - Page 148: 18

$20\sin x(cos^{-5}x)$

#### Work Step by Step

With $u=\cos x$ and $y=5u^{-4}$: $\frac{dy}{dx}=\frac{dy}{du}\cdot\frac{du}{dx}=5\times-4\times u^{-5}\times-\sin x= 20\sin x(cos^{-5}x)$

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