Question 151528
Starting with the given equation:{{{h = -16t^2+8256}}}, you want to find at what time, t, will h = 8000.
{{{8000 = -16t^2+8256}}} Subtract 8000 from both sides.
{{{-16t^2+256 = 0}}} Multiply through by -1.
{{{16t^2-256 = 0}}} Notice that the binomial is the difference of squares, so you can factor this nicely...
{{{(4t-16)(4t+16) = 0}}} Now apply the zero products rule.
{{{4t-16 = 0}}} or {{{4t+16 = 0}}}
If {{{4t-16 = 0}}} then{{{4t = 16}}} and {{{t = 4}}}
If {{{4t+16 = 0}}} then {{{4t = -16}}} and {{{t = -4}}}
Discard the negative answer because the time, t, should be positive.
It take 4 second for the skydiver to reach a height of 8,000 feet.