Originally Posted by

**tedhill** I started to learn calculus following a book but I don’t understand the explanation of a derivative example. Here is the excerpt of the book:

**Then the Pythagorean theorem tells us that**

**h(t)^2 + b(t)^2 = 13^2**

**We may differentiate both sides of this equation with respect to the variable t**

**(which is time in minutes) to obtain**

**2 · h(t) · h’(t) + 2 · b(t) · b’(t) = 0**

It seems that the author applied the Chain Rule but I am not sure about the intermediary steps of this differentiation. Could someone show me the intermediary steps and explain them if needed ?