A projectile is thrown upward so that its distance above the ground after $t$ sec is given by $h(t)=-13 t^{2}+390 t$. After how many seconds does it reach its maximum height?
A) $7 \mathrm{sec}$
B) $15 \mathrm{sec}$
C) $30 \mathrm{sec}$
D) $22.5 \mathrm{sec}$

Question

Quizplus · Accepted Answer

The maximum height of a projectile in a quadratic equation of the form $h(t) = at^2 + bt + c$ is found at $t = -\frac{b}{2a}$. Here, $a = -13$ and $b = 390$, so $t = -\frac{390}{2(-13)} = 15$ seconds.

A Projectile Is Thrown Upward So That Its Distance Above ttt

A Projectile Is Thrown Upward So That Its Distance Above $t$