Question

A ball is thrown in the air from a ledge. It's height in feet represented by f(x)=16(x^2-6x-7), where x is the number of seconds since the ball has been thrown. The height of the ball is 0 feet when it hits the ground. How many seconds does it take the ball to reach the ground?

Answer 1

7 seconds.

Explanation:

height h = 16(x^2-6x-7) = 0

x^2 - 6x - 7 = 0

(x - 7)(x + 1) = 0

x = 7 seconds (we ignore the negative).

Answer 2

Explanation:

Since we know that the height is 0, we can figure out how long it took the ball to reach the ground by setting
`f(x) = 0` and solving for
`x`:

`f(x) = 16(x^(2) - 6x - 7)`

`0 = 16(x^(2) - 6x - 7)`

`0 = x^(2) - 6x - 7`

`0 = (x - 7)(x + 1)`

`x = -1, 7`

Because time can only be positive, the answer is 7 seconds.