Mathematics

Mathematics

Problem

Simplify when possible. The answers are in Chapter 23, starting on page 464. 18. Find dydt\frac{d y}{d t} if y=(x6−6x5)(5x2+x)y=\left(x^{6}-6 x^{5}\right)\left(5 x^{2}+x\right) and x=tx=\sqrt{t}.
Intuitive explanation

This is a chain rule problem disguised as a product. You have yy depending on xx, which depends on tt, so you need dydt=dydx⋅dxdt\frac{dy}{dt} = \frac{dy}{dx} \cdot \frac{dx}{dt}. The key insight is to use the product rule to find dydx\frac{dy}{dx} first, then multiply by the derivative of x=tx = \sqrt{t} with respect to tt. Substituting x=tx = \sqrt{t} at the end will give you your final answer in terms of tt alone.

Intuitive explanation

This is a chain rule problem disguised as a product. You have yy depending on xx, which depends on tt, so you need dydt=dydx⋅dxdt\frac{dy}{dt} = \frac{dy}{dx} \cdot \frac{dx}{dt}. The key insight is to use the product rule to find dydx\frac{dy}{dx} first, then multiply by the derivative of x=tx = \sqrt{t} with respect to tt. Substituting x=tx = \sqrt{t} at the end will give you your final answer in terms of tt alone.

Browse the problem library