Alain throws a stone off a bridge into a river below.

Question

The stone's height (in meters above the water), x seconds after Alain threw it, is modeled by:

h(x)=−5x^2 + 10x + 15

How many seconds after being thrown will the stone hit the water?

Answer 1

Set h(x) = 0 and solve for x using the quadratic formula (which you should be familiar with), or by factoring. You will get a negative and a positive solution. Since time starts at x=0, the positive solution is the correct answer.