<PRE><B>Question 80468</B>
Use the formula h=-16t^2+250t to model the height h in feet of a model rocket t seconds after it is launched.Determine when the rocket will reach a height of 900 feet. 
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Substitute 900 for h:
:
-16t^2 + 250t = 900
:
-16t^2 + 250t - 900 = 0; a quadratic equation
:
Use the quadratic formula: a=-16; b=250; c=-900
{{{t = (-250 +- sqrt( 250^2-4*-16*-900 ))/(2*-16) }}}
:
do the math and you should get two positive solutions giving us the times when
rocket will be at 900 ft:
:
t = 5.625 sec (on the way up) and t = 10 sec (on the way down</PRE>