  ### Home > GC > Chapter 12 > Lesson 12.2.4 > Problem12-87

12-87.
1. Perry threw a tennis ball up into the air from the edge of a cliff. The height of the ball was y = −16xs2 + 64x + 80 , where y represents the height in feet of the ball above ground at the bottom of the cliff, and x represents the time in seconds after the ball is thrown. Homework Help ✎

1. How high was the ball when it was thrown? How do you know?

2. What was the height of the ball 3 seconds after it was thrown? What was its height a second after it was thrown? Show all work.

3. When did the ball hit the ground? Write and solve an equation that represents this situation. Substitute 0 in for x.

y = -16(0²) + 64(0) + 80 = 80 feet

Substitute 3 for x in the given equation.

y = -16(3²) + 64(3) + 80
y = -144 + 192 + 80
y = -144 + 272
y = 128 ft

y = -16(0.5²) + 64(0.5) + 80
y = -4 + 32 + 80
y = -4 + 112
y = 108 ft

If the ball hits the ground, what is its height above the ground?

-16x² + 64x + 80 = 0

−16(x² − 4x − 5)
−16(x − 5)(x + 1)
x = 5 seconds