### Home > CALC > Chapter 7 > Lesson 7.2.2 > Problem7-71

7-71.

Remember Eric and the $16$-foot tall lamppost? If Eric (who is $5$ feet tall) walks away from the pole at a rate of $4\frac {\text{ ft }} { \operatorname { cos } }$, at what rate is the tip of his shadow moving away from the lamppost? Draw a diagram before you start this problem.

Find similar triangles and write a geometric equation relating their proportional sides.

Use implicit differentiation to convert the geometric equation into a rate equation.
In other words, find the derivative of everything in terms of time, $t$.