Question

Benjamin threw a rock straight up from a cliff that was 48 ft above the water. If the height of the rock​ h, in​ feet, after t seconds is given by the equation h = -16 t² + 52 t + 48, how long will it take for the rock to hit the​ water?

Answer 1

Answer: It will take 4 seconds  for the rock to hit the​ water.

Step-by-step explanation:

Given : Benjamin threw a rock straight up from a cliff that was 48 ft above the water.

If the height of the rock​ h, in​ feet, after t seconds is given by the equation

$h = -16 t^2+ 52 t + 48$

To find : Time taken for the rock to hit the​ water.

When rock hit the water height becomes zero , i.e. put h = 0 , we get

$-16 t^2+ 52 t + 48 =0$

Divide equation by 2 , we get

$-8t^2+26t+24=0$

The Laue of x for ax²+bx+c =0 is $x=\dfrac{-b\pm \sqrt{b^2-4ac}}{2a}$

So , $t=\frac{-26\pm\sqrt{(26^{2}-4(-8)(24))}}{2(-8)}$

$t=\dfrac{-26\pm 38}{-16}\\\\ t=-\dfrac{-26+38}{-16}=-0.75\ \ t=\dfrac{-26-38}{-16}=4$

Since t cannot be negative , so t= 4

Hence, it will take 4 seconds  for the rock to hit the​ water.