<PRE><B>Question 78382</B>
h = -5t^2 + 20t +100
37. Science and medicine. How long does it take the ball to fall back to the ground?

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The height is zero when the ball is back on the ground.
Solve -5t^2+20t+100=0
t^2-4t-20=0
t=[4+-sqrt(16-4*-20)]/2
t=[4+-sqrt(96)]/2
t=[4+-4sqrt6)[/2
t=[2+-2sqrt6]
The positive answer is:
t=2+2sqrt6 = 6.9 seconds
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{{{graph(400,300,0,10,-2,150,-3x^2+20x+100)}}}

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Cheers,
Stan H,
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Cheers,
Stan H.</PRE>