A baseball is hit from a height of 0.8 m. After “t” seconds, its height “h”, in metres, is given by the equation h = -5t2 + 20t + 0.8. How long does the ball stay at least 15 m above the ground?

Question

A baseball is hit from a height of 0.8 m. After “t” seconds, its height “h”, in metres, is given by the equation h = -5t2 + 20t + 0.8. How long does the ball stay at least 15 m above the ground?
 
What would be the LET statement?

Reiny · Answer

Let
-5t2 + 20t + 0.8 > 15
5t^2 - 20t + 14.2 < 0

You know that y = 5t^2 - 20t + 14.2 is a parabola opening upwards
find the roots of 5t^2 - 20t + 14.2 = 0 , and the time will be the time between those two roots