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\begin{align*}\begin{aligned}&=\left(\lim _{\Delta x \rightarrow 0} \frac{\Delta y}{\Delta u}\right) \cdot\left(\lim _{\Delta x \rightarrow 0} \frac{\Delta u}{\Delta x}\right) \quad u \text { is continuous, so } \Delta x \rightarrow 0 \text { implies } \Delta u \rightarrow 0 \\&=\frac{d y}{d u} \cdot \frac{d u}{d x}\end{aligned}\end{align*}