Question 72977
an object is projected upward from the top of 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.
It's height will be 0 when it hits the ground, so set the problem = to 0.
{{{0=-16t^2+64t+80}}}
{{{0=-16(t^2-4t-5)}}}
{{{0/-16=-16(t^2-4t-5)/-16}}}
{{{0=t^2-4t-5}}}  Factor
{{{0=(t-5)(t+1)}}}  set each factor = to 0 and solve for t.
t-5=0 or t+1=0
t-5+5=0+5 or t+1-1=0-1
t=5 or t=-1
Negative time is meaningless to us, so t=5 seconds.
Happy Calculating!!!!