math
posted by miguel .
A smartphone is thrown upwards from the top of a 240foot building with an initial velocity of 32 feet per second. The height h of the smartphone after t seconds is given by the quadratic equation h=16t^2+32t+240. When will the smartphone hit the ground?

when it hits the ground, h = 0
so solve:
16t^2  32t  240 = 0
t^2  2t  15 = 0
(t5)(t+3) = 0
t = 5 or t = 3, which is not acceptable
it will hit the ground after 5 seconds