# Thread: Solve by completing the square

1. ## Solve by completing the square

A stone is thrown in the air. After t seconds its height, h, above sea level is given by the formula h=80+3t-5t^2. Find the value of t when the stone falls into the sea.

Thank you!

2. Originally Posted by Anna55

A stone is thrown in the air. After t seconds its height, h, above sea level is given by the formula h=80+3t-5t^2. Find the value of t when the stone falls into the sea.

Thank you!
What is the value of h when the stone hits the sea? Therefore, what equation do you need to solve? Make an attempt at solving it - show all your work and say where you get stuck.

3. I rearranged the equation like this. -5t^2+3t+80=0 I tried to solve it by completing the square but the answer was incorrect. I am not sure if the equation should equal 0.

4. Originally Posted by Anna55
I rearranged the equation like this. -5t^2+3t+80=0 I tried to solve it by completing the square but the answer was incorrect. I am not sure if the equation should equal 0.
That is the correct equation.

Are you totally certain it has to be solved by completing the square? It would be much more efficient to use the quadratic formula.

5. Yes, you can also solve it with the quadratic formula. Thank you for the advice!