A projectile is thrown upward so that its distance above the ground after t seconds is h =-13t2 + 390t. After how many seconds does it reach its maximum height? A) 22.5 s B) 7 s C) 15 s D) 30 s

Step-by-step explanation:

Given equation is

Given that a projectile is thrown upward so that its distance above the ground after t seconds is

"The standard form of a parabola's equation is  expressed as :

.

If , then the parabola opens upwards;

if the parabola opens downwards."

The maximum height is the vertex of the parabola   which is .

Comparing the given equation with the standard form of parabola we get

the values of a=-13 , b=390 and c=0

The maximum height in .

Substitute the values we get

one that is bigger than 3 1/2