Answer:
0.46 seconds or 11.72 seconds
Step-by-step explanation:
Putting 87 for h in the height equation gives ...
87 = -16t^2 +195t
Dividing by -16 puts this in a little nicer form for finding solutions.
t^2 -(195/16)t = -87/16
To "complete the square", we can add the square of half the coefficient of t:
t^2 -(195/16)t + (195/32)^2 = (195/32)^2 -87/16
(t -195/32)^2 = 32457/1024
Taking the square root, we have ...
t -195/32 = ±(1/32)√32457
t = (195 ±√32457)/32 ≈ 0.4638 or 11.7237
The rocket is at a height of 87 feet after 0.46 seconds, and again after 11.72 seconds.
