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7-66.

Remember Eric and the -foot tall lamppost? If Eric (who is feet tall) walks away from the lamppost at a rate of ft/sec, at what rate is the tip of his shadow moving away from the lamppost? Draw a diagram before you start this problem.

Use similar triangles to write an equation relating the 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, .