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Use the direction in which the distance from the street light increases as the positive direction.
Solution
Get a formula for the shadow’s length in terms of time
The shadow’s length = x in feet
t= time in seconds
7/x = 18/(x+15-6t) ratios of two sides of similar triangles are equal
solve for x in terms of t; cross multiply
11x = 105 – 42t
x = 105/11 -42t/11
take derivative
dx/dt = -42/11 feet per second = – 3 9/11 ft/sec
The shadow is getting shorter by almost 4 feet every second