document.write( "Question 946130: Benjamin threw a rock straight up from a cliff that was 100 ft above the water. If the height of the rock h, in feet, after t seconds is given by the equation h=-16t^2 +60t +100 how long will it take for the rock to hit the water? \n" ); document.write( "
Algebra.Com's Answer #577200 by Edwin McCravy(20056)\"\" \"About 
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document.write( "         ·h = 126 ft. highest point rock reaches before it starts to fall\r\n" );
document.write( "         |            [you don't need to know it is 126 ft. (vertex formula)]\r\n" );
document.write( "cliff    |\r\n" );
document.write( "---------·h = 100 ft  (cliff level)\r\n" );
document.write( "         |\r\n" );
document.write( "         |\r\n" );
document.write( "         |  \r\n" );
document.write( "         |\r\n" );
document.write( "---------·h = 0 ft.  (water level)\r\n" );
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document.write( "Since the water level is 0, we set h=9 and solve for t:\r\n" );
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document.write( "h = -16t² + 60t + 100 \r\n" );
document.write( "0 = -16t² + 60t + 100\r\n" );
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document.write( "divide both sides by -4\r\n" );
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document.write( "0 = 4t² - 15t - 25\r\n" );
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document.write( "0 = (2t+5)(t-5)\r\n" );
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document.write( "   0 = 2t+5;  0 = t-5\r\n" );
document.write( "  -5 = 2t;    5 = t\r\n" );
document.write( "-5/2 = t;\r\n" );
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document.write( "Ignore the negative answer.\r\n" );
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document.write( "Answer: 5 seconds.\r\n" );
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document.write( "Edwin
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