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The shape of an entrance to a tunnel can be modeled by the graph of the equation y = -0.2x(x - 20) where x and y are measured in feet. On a coordinate plane, the ground is represented by the x-axis. How wide is the tunnel at its base? Explain how you found your answer.

 Apr 7, 2018
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Since we know it is an upside down parablola, the two points where it crosses the x axis (where y=0) will define the width

so

y = 0 = -0.2 x  (x-20)       means   x = 0    and x = 20   are the places where it crosses the x-axis (ground level)

distance between 0 and 20 = 20 ft

 Apr 7, 2018

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