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  1. A 4-foot tall child walks away from a 12-foot street light at a rate of 2 feet per second. At what rate is the length of her shadow increasing when she is 6 feet away from the light? Homework Help ✎

    1. 1 ft/sec

    2. 2 ft/sec

    3. 3 ft/sec

    4. 4 ft/sec

    5. 6 ft/sec

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

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

The equation can be rewritten as 2s = d.
Now differentiate.

Now use the information given in the problem to solve for ds/dt.