### Home > CALC > Chapter 7 > Lesson 7.1.5 > Problem 7-51

7-51.

Start with the geometric equation of the area within a circle. There is no Calculus involved in this step.

*A* = π*r*²

Do NOT substitute a value for *r*, because the *r* changes. That is where Calculus comes in...

Implicitly differentiate with respect to time, *t*.