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)![]() ![]() You can put this solution on YOUR website! 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( ": \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( " |