CPM Homework Banner

Multiple Choice: A -foot tall child walks away from a ‑foot tall street light at a rate of ft/sec. At what rate is the length of her shadow increasing when she is feet away from the light? Homework Help ✎

  1. 1 ft/sec

  1. 2 ft/sec

  1. 3 ft/sec

  1. 4 ft/sec

  1. 6 ft/sec

Sketch the situation. Your sketch should have similar right triangles.

If the child's distance from the light is and their shadow length is , then:

The equation can be rewritten as .
Now differentiate.

Now use the information given in the problem to solve for .