CPM Homework Help : CALC Problem 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.

Diagram:

Step 1:

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

Step 2:

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