9514 1404 393
Explanation:
To find the time, substitute the height for h(t), then solve the quadratic equation for t.
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To solve the equation, you subtract the height from both sides, then use the quadratic formula. In generic terms, this looks like ...
 -16t^2 +32t -h = 0
 t = (-32±√(32^2 -4(-16)(-h)))/(2(-16))
 t = (-32 ± √(1024-64h))/-32
 t = 1 ± √(1 -h/16)
For h = 10 ft, the times are ...
 t = 1 ± √(3/8) ≈ {0.388, 1.612} . . . . seconds
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Using the quadratic formula is not the only way to solve a quadratic. One of my favorite methods is using a graphing calculator.
