document.write( "Question 43505This question is from textbook Algebra and trigonometry with analytic geometry
\n" ); document.write( ": An object is projected upward from the top of a water tower. Its distance in feet above the ground after t seconds is given by s(t)=-16t^2+64t+80. How many seconds will it take to reach ground level? I came out with 4 seconds after working it out. thanks for checking. \n" ); document.write( "
Algebra.Com's Answer #28502 by AnlytcPhil(1810)\"\" \"About 
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document.write( "An object is projected upward from the top of a water \r\n" );
document.write( "tower. Its distance in feet above the ground after t \r\n" );
document.write( "seconds is given by s(t) = -16t^2 + 64t + 80. How \r\n" );
document.write( "many seconds will it take to reach ground level? I \r\n" );
document.write( "came out with 4 seconds after working it out. thanks \r\n" );
document.write( "for checking.\r\n" );
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document.write( "Let's check your answer and see if it is correct:\r\n" );
document.write( "We substitute 4 seconds into the equation:\r\n" );
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document.write( "                     s(t) = -16t² + 64t + 80              \r\n" );
document.write( "                     s(4) = -16(4)² + 64(4) + 80                      \r\n" );
document.write( "                     s(4) = -16(16) + 256 + 80\r\n" );
document.write( "                     s(4) = -256 + 256 + 80\r\n" );
document.write( "                     s(4) = 80 feet above ground\r\n" );
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document.write( "After 4 seconds the object is still 80 feet above \r\n" );
document.write( "the ground.  So 4 seconds can't be right. Let's\r\n" );
document.write( "do it right:\r\n" );
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document.write( "Ground level is ground zero, that is, when the\r\n" );
document.write( "height above the ground s(t) is zero,\r\n" );
document.write( "So replace s(t) by 0\r\n" );
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document.write( "                     s(t) = -16t² + 64t + 80\r\n" );
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document.write( "                        0 = -16t² + 64t + 80\r\n" );
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document.write( "          16t² - 64t - 80 = 0\r\n" );
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document.write( "Divide every term through by 16\r\n" );
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document.write( "              t² - 4t - 5 = 0\r\n" );
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document.write( "           (t - 5)(t + 1) = 0 \r\n" );
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document.write( "          t - 5 = 0;  t + 1 = 0\r\n" );
document.write( "              t = 5       t = -1\r\n" );
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document.write( "Answer: 5 seconds.\r\n" );
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document.write( "Discard the negative answer. It takes 5 seconds\r\n" );
document.write( "to reach the ground.  The water tank is 80 feet high.\r\n" );
document.write( "At the instant when 2 seconds have passed it reaches\r\n" );
document.write( "its maximum height of 144 feet (64 feet above the \r\n" );
document.write( "water tower). Then it starts falling back down.  At\r\n" );
document.write( "the instant when 4 seconds have passed, it is back\r\n" );
document.write( "even with the tower.  Then after 5 seconds have\r\n" );
document.write( "passed the object reaches the ground.\r\n" );
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document.write( "Edwin\r\n" );
document.write( "AnlytcPhil@aol.com
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