Question 229227
{{{h=80+64t-16t^2}}}
After how many seconds will it hit the water (h=0)?
:
{{{80 + 64t - 16t^2}}} = 0
normal form
{{{-16t^2 + 64t + 80}}} = 0
Factor out -16, simplifies and changes the signs
-16(t^2 - 4t - 5) = 0
Factors to
-16(t - 5)(t + 1) = 0
The positive solution
t - 5 = 0
t = +5 sec to hit the water
;
:
prove this to yourself
{{{h = 80 + 64(5) - 16(5^2)}}}
h = 80 + 320 - 16(25)
h  = 400 - 400
h = 0