Question

The quadratic function

d=-9x² + 40 models a
skateboarder's distance, in
feet, from the bottom of a hill
x seconds after the skateboarder
starts moving down the hill.
After how many seconds is the
skateboarder 4 ft from the
bottom of the hill?

A) 3s B) 4s C) 2s D) 3.5s

Please help!! Also if you could explain that would be great! Tysm!

Answer 1

C

Explanation:

equate d to 4 and solve for t , that is

- 9x² + 40 = 4 ( subtract 40 from both sides )

- 9x² = - 36 ( divide both sides by - 9 )

x² = 4 ( take square root of both sides )

x =
`√(4)` = 2

It takes 2 seconds for the skateboarder to be 4m from bottom of hill