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

The option C) 15 s is correctTherefore the maximum height it reaches in 15 seconds.

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

To find how many seconds does it reach its maximum height:

"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

∴ t=15 s∴ The option C) 15 s is correct.∴ the maximum height is the vertex of the parabola, at t reaches in 15 seconds.

Will start to distribute the 2 in (x-7)
so it can be (2x-2*7)=100
(2x-14)=100
2x=100+14
2x=114
x=114/2
x=57