SOLUTION: Hello! I have a word problem and I have no idea how to solve it, if you could solve it and show me the steps: A rock is thrown from a top of the tall building. The distance,in fee

Question 553584: Hello! I have a word problem and I have no idea how to solve it, if you could solve it and show me the steps:
A rock is thrown from a top of the tall building. The distance,in feet, between the rock and the ground t seconds after it is thrown is given by d= -16t^2-4t+412. How long after the rock is thrown is it 400 feet from the ground? (answer should be in decimal form)
Answer by Earlsdon(6294) (Show Source): You can put this solution on YOUR website!
Start with:
You are being asked to solve this equation for t when d (distance from the ground) is 400, so you substitute d = 400.
Subtract 400 from both sides.
Factor -4 to make it a little easier to solve.
Factor the trinomial.
so now you have:
Apply the zero product rule.
or or
No way!
so and or
so Discard this solution as the time must be a positive quantity.
The rock will be 400 feet from the ground in 0.75 seconds.

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