document.write( "Question 79829: A ball is thrown upward from the roof of a building 100 m tall with an initial velocity of 20 m/s. When will the ball reach a height of 80 m? \n" ); document.write( "
Algebra.Com's Answer #57287 by ankor@dixie-net.com(22740)\"\" \"About 
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A ball is thrown upward from the roof of a building 100 m tall with an initial velocity of 20 m/s. When will the ball reach a height of 80 m?
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\n" ); document.write( "Use the following equation:
\n" ); document.write( "-16t^ + 20t + 100 = h
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\n" ); document.write( "-16t^2 + 20t + 100 = 80
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\n" ); document.write( "-16t^2 + 20t + 100 - 80 = 0
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\n" ); document.write( "-16t^2 + 20t + 20 = 0
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\n" ); document.write( "Simplify, divide equation by -4 and you have:
\n" ); document.write( "+4t^2 - 5t - 5 = 0
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\n" ); document.write( "Use the quadratic formula to find t: a=4; b=-5; c=-5
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\n" ); document.write( "I got t = -.655; and t = +1.9 sec is the solution we want:
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\n" ); document.write( "Check solution in original equations:
\n" ); document.write( "h - -16t^2 + 20x + 100
\n" ); document.write( "h = -16(1.9^2) + 20(1.9) + 100
\n" ); document.write( "h = -16(3.61) + 38 + 100
\n" ); document.write( "h = -57.56
\n" ); document.write( "h = 80.24 ~ 80 ft\r
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