A ball is thrown from a height of 20 meters with an initial downward velocity of 5 m/s. The ball's height h (in meters) after t seconds is given by the following.
h=20-5t-5t²
How long after the ball is thrown does it hit the ground?
Round your answer(s) to the nearest hundredth.
(If there is more than one answer, use the "or" button.)

1.56 seconds

Step-by-step explanation:

When the ball hits the ground, h = 0.

0 = 20 − 5t − 5t²

Divide both sides by -5.

0 = t² + t − 4

Solve with quadratic formula.

t = [ -1 ± √(1² − 4(1)(-4)) ] / 2(1)

t = (-1 ± √17) / 2

The time must be positive, so:

t = (-1 + √17) / 2

t ≈ 1.56

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step-by-step explanation: