SOLUTION: the height h (in feet) above the ground of a stone tossed over a bridge is given by h= -16t^2+20t+160 , where t is the time (in seconds) since the stone was tossed. how long, to

Algebra ->  Rational-functions -> SOLUTION: the height h (in feet) above the ground of a stone tossed over a bridge is given by h= -16t^2+20t+160 , where t is the time (in seconds) since the stone was tossed. how long, to       Log On


   

Question 964507: the height h (in feet) above the ground of a stone tossed over a bridge is given by h= -16t^2+20t+160 , where t is the time (in seconds) since the stone was tossed. how long, to the nearest hundredth of a second, will it take until the stone is at a height of 40 feet above the ground?
Answer by lwsshak3(11628) About Me  (Show Source):
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the height h (in feet) above the ground of a stone tossed over a bridge is given by h= -16t^2+20t+160 , where t is the time (in seconds) since the stone was tossed. how long, to the nearest hundredth of a second, will it take until the stone is at a height of 40 feet above the ground?
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h= -16t^2+20t+160
40= -16t^2+20t+160
-16t^2+20t+120=0
divide by 4:
-4t^2+5t+30=0
solve for t by quadratic formula:
t+=+%28-b+%2B-+sqrt%28+b%5E2-4%2Aa%2Ac+%29%29%2F%282%2Aa%29+
a=-4, b=5, c=30
ans:
t=3.43
how long will it take until the stone is at a height of 40 feet above the ground? 3.43 sec