Posted by Emily on Thursday, June 20, 2013 at 3:16pm.
"A rock is dropped from the edge of a 180 m cliff. the function f(x) = 5t^2  5t + 180 gives the height of the rock, h(t), in meters t seconds after it was released. How long does it take for the rock to reach a ledge 80 m above the base of the cliff?"
I have no idea how to solve this, steps and explanations would be amazing.

Math Gr 11  Steve, Thursday, June 20, 2013 at 3:51pm
well, since f(t) is the height of the rock at time t, just set it to 80 and solve for t:
5t^2  5t + 180 = 80
5t^2 + 5t  100 = 0
t^2 + t  20 = 0
(t+5)(t4) = 20
t=4
check:
f(4) = 5(16)5(4)+180 = 80
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