1. The problem is to state the formula for the chain rule using $\frac{dy}{dx}$. 2. The chain rule is used to differentiate composite functions. If $y$ is a function of $u$, and $u$ is a function of $x$, then $y$ is a function of $x$ through $u$. 3. The formula for the chain rule is: $$\frac{dy}{dx} = \frac{dy}{du} \times \frac{du}{dx}$$ 4. This means to find the derivative of $y$ with respect to $x$, you multiply the derivative of $y$ with respect to $u$ by the derivative of $u$ with respect to $x$. 5. Important rule: Always identify the inner function $u(x)$ and the outer function $y(u)$ before applying the chain rule. 6. This formula helps differentiate complex functions by breaking them into simpler parts. Final answer: $$\frac{dy}{dx} = \frac{dy}{du} \times \frac{du}{dx}$$