9514 1404 393
Answer:
5.04 seconds
Step-by-step explanation:
Assuming the ball is in the air until f(x) = 0, we want to find x such that ...
0 = -5x² +25x +1
0 = x² -5x -1/5 . . . . . divide by -5
1/5 = x² -5x . . . . . . . add 1/5
1/5 +25/4 = (x² -5x +25/4) . . . . complete the square
129/20 = (x -5/2)² . . . . . . . . . . write as a square
x = 5/2 +√(129/20) ≈ 5.039685 . . . . square root, add 5/2
The ball stays in the air about 5.04 seconds.
