Question 59420
An object is projected upward from the top of a 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?
let s(t)=0  that's at ground level.
{{{0=-16t^2+64t+80}}}
{{{0/-16=-16t^2/-16+64t/-16+80/-16}}}
{{{0=t^2-4t-5}}}  factor
0=(t+1)(t-5)
t+1=0 and t-5=0
t+1-1=0-1 and t-5+5=0+5
t=-1 and t=5
Negative time is not valid.
Therefore {{{highlight(t=5s)}}}
Happy Calculating!!!