SOLUTION: Word Problem Help A ball is thrown from an initial height of 4 feet with an initial upward velocity of 31 ft/s. The ball's height h (in feet) after t seconds is given by the fol

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Question 1181268: Word Problem Help
A ball is thrown from an initial height of 4 feet with an initial upward velocity of 31 ft/s. The ball's height h (in feet) after t seconds is given by the following.
+h=4%2B31t-16t%5E2+

Find all values of for which the ball's height is 18 feet.
Round your answer(s) to the nearest hundredth.
Answer by MathLover1(20850) About Me  (Show Source):
You can put this solution on YOUR website!

if h=18 feet
+18=4%2B31t-16t%5E2+.......solve for t
+0=-18%2B4%2B31t-16t%5E2+
+0=-14%2B31t-16t%5E2+
+0=-16t%5E2%2B31t+-14
use quadratic formula to calculate t
t+=+%28-b+%2B-+sqrt%28+b%5E2-4%2Aa%2Ac+%29%29%2F%282%2Aa%29+.....you have a=-16,b=31,c=-14
t+=+%28-31+%2B-+sqrt%28+31%5E2-4%2A%28-16%29%2A%28-14%29+%29%29%2F%282%2A%28-16%29%29+
t+=+%28-31+%2B-+sqrt%28+961-896+%29%29%2F%28-32%29+
t+=+%28-31+%2B-+sqrt%28+65+%29%29%2F%28-32%29+

solutions:
t+=+%28-31+%2B+sqrt%28+65+%29%29%2F%28-32%29+=>t+=31%2F32+-+sqrt%28+65+%29%2F32+=>t=0.72
t+=+%28-31+-+sqrt%28+65+%29%29%2F%28-32%29+=> t+=+31%2F32+%2B+sqrt%28+65+%29%2F32+=>t=1.22