Question

A rock is thrown and follows the curve given by the equation d = -t2 + 4t + 5, where d is the distance in feet and t is the time in seconds. When will the rock hit the ground? 9 seconds 5 seconds 2 seconds 1 second

Answer 1

5 seconds

Explanation:

The relationship between "d" and "the ground" is not described. If we assume that "d" is distance above the ground, then the rock will hit the ground when d=0. This gives rise to the quadratic equation ...

-t^2 +4t +5 = 0

-(t -5)(t +1) = 0

t = 5 or t = -1 are solutions. Only the positive solution is useful.

The rock will hit the ground after 5 seconds.