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

answer:   28.56 feet (approx)

step-by-step explanation:

since, triangle abc has measure of angle c equal to 55 degrees, measure of angle abc equal to 90 degrees, and length of bc equal to 20 feet.

we have to find out the measurement of segment ab.

therefore, tan 55° =

but bc = 20 feet.

⇒ tan 55 ° =  

⇒ ab = 20 × tan 55 °

⇒ ab = 20 × 1.42814800674

⇒ ab = 28.5629601348   ≈ 28.56 feet