document.write( "Question 945132: Benjamin threw a rock straight up from a cliff that was 120 ft above the water. If the height of the rock h, in feet, after t seconds is given by the equation h=-16t^2+76t+120, how long will it take for the rock to hit the water? \n" ); document.write( "

Algebra.Com's Answer #576310 by ankor@dixie-net.com(22740) $\"\"$ $\"About$

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Benjamin threw a rock straight up from a cliff that was 120 ft above the water. If the height of the rock h, in feet, after t seconds is given by the equation h=-16t^2+76t+120, how long will it take for the rock to hit the water?
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\n" ); document.write( "When the rock hits the water it's height = 0, therefore:
\n" ); document.write( "-16t^2 + 76t + 120 = 0
\n" ); document.write( "simplify, divide equation by -4
\n" ); document.write( "4t^2 - 19t - 30 = 0
\n" ); document.write( "we can use the quadratic formula to solve this, a=4; b=-19; c=-30;
\n" ); document.write( "but this will factor to
\n" ); document.write( "(4t + 5)(x - 6) = 0
\n" ); document.write( "the positive solution is all we want here
\n" ); document.write( "t = 6 sec to hit the water
\n" ); document.write( ":
\n" ); document.write( ":
\n" ); document.write( "Confirm this for yourself, calculate
\n" ); document.write( "h = -16(6^2) + 76(6) + 120 \n" ); document.write( "

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