Posted by **Tina** on Saturday, July 16, 2011 at 10:19pm.

A person standing near the edge of a cliff 120 feet above a lake throws a rock upward with an initial speed of 40 feet per second. The height of the rock above the lake at the bottom of the cliff is a function of time and is described by the function H(t) = –16t2 + 40t + 120.

a) How many seconds will it take until the rock reaches its maximum height?

b) What is that height?

- Math -
**Max**, Saturday, July 16, 2011 at 11:03pm
H(t) = -16t^2 + 40t + 120

Finding critical point...

H'(t) = -32t + 40 ...... Differentiation

0 = -32t + 40

32t = 40

t = 1.25 seconds

Thats you answer for (a). Use it to find the answer for (b).

- Math - typo -
**Max**, Saturday, July 16, 2011 at 11:04pm
Thats you...

I meant "that's your..."

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