SOLUTION: Robert threw a rock off a bridge into the river. The distance from the rock to the river is molded by the equation h=-16t^2-16t+60, where h is the height in ft. and t is the time i

Algebra ->  Quadratic Equations and Parabolas  -> Quadratic Equations Lessons  -> Quadratic Equation Lesson -> SOLUTION: Robert threw a rock off a bridge into the river. The distance from the rock to the river is molded by the equation h=-16t^2-16t+60, where h is the height in ft. and t is the time i      Log On


   

Question 1154978: Robert threw a rock off a bridge into the river. The distance from the rock to the river is molded by the equation h=-16t^2-16t+60, where h is the height in ft. and t is the time in seconds. find out how long it took the rock to hit the surface. solve the equation by factoring.
Answer by MathLover1(20849) About Me  (Show Source):
You can put this solution on YOUR website!
h=-16t%5E2-16t%2B60, where h is the height in ft and t is the time in seconds
the rock will hit the surface when h=0
to find how long it will took the rock will hit the surface,plug in h=0 and solve for t
0=-16t%5E2-16t%2B60........factor out -4
0=-4%284t%5E2%2B4t-15%29...write 4t as 10t-6t
0=-4%284t%5E2%2B10t-6t-15%29
0=-4%28%284t%5E2%2B10t%29-%286t%2B15%29%29
0=-4%282t%282t%2B5%29-3%282t%2B5%29%29
0=-4%282t+-3%29%282t+%2B+5%29
solutions:

if 0=%282t+-3%29=>2t=3=>t=3%2F2=>t=1.5
if 0=+%282t+%2B+5%29=>2t=-5=>t=-5%2F2=> disregard negative solution

so, it took 1.5 seconds the rock to hit the surface