Solve the problem.
-A projectile is thrown upward so that its distance above the ground after $t$ seconds is $h = - 16 t ^ { 2 } + 640 t$ After how many seconds does it reach its maximum height?
A) $10 \mathrm {~s}$
B) $30 \mathrm {~s}$
C) $40 \mathrm {~s}$
D) $20 \mathrm {~s}$

Question

Quizplus · Accepted Answer

The maximum height of a projectile is reached at the vertex of the parabola described by the equation $h = -16t^2 + 640t$. The vertex occurs at $t = -\frac{b}{2a}$, where $a = -16$ and $b = 640$. Calculating gives $t = -\frac{640}{2 \times -16} = 20$ seconds.

Solve the Problem ttt Seconds Is h=−16t2+640th = - 16 t ^ { 2 } + 640 th=−16t2+640t

Solve the Problem $t$ Seconds Is $h = - 16 t ^ { 2 } + 640 t$