For any two differentiable functions, the derivative of the product of two functions is the first function times the derivative of the second plus the second function times the derivative of the first.
The derivative of a polynomial is the sum of the derivatives of its terms. The derivative of a constant term is $0$. The derivative of $ax^{n}$ is $nax^{n-1}$.
$$x^{4}\times 5x^{5-1}+x^{5}\times 4x^{4-1}$$
Simplify.
$$x^{4}\times 5x^{4}+x^{5}\times 4x^{3}$$
To multiply powers of the same base, add their exponents.
