Question

The path traveled by a bottlenose dolphin as it jumps out of water is modeled by the equation y = −0.4x2 + 3x, where y is the height above water and x is the horizontal distance in feet. If a beam of light is shone upward at an angle modeled by the equation x + y = 10, at what height from the water's surface will the beam of light hit the dolphin?
A)2.7 feet

B)3 feet

C)5 feet

D)5.6 feet

Answer 1

We can find the answer by solving the two equations simultaneously.

equation 1: y = -0.4x^2 +3xequation 2: x + y = 10
We need to write equation to with y as the subject:equation 2: y = 10 - x

equation 1 = equation 2:
10 - x = -0.4x^2 + 3x0.4x^2 - 4x + 10 = 02x^2 - 20 + 50 = 0(2x - 10)(x - 5) = 0x = -10/2 or x = 5

But x >= 0 (the question says it's a height above the water), so x = 5

Substituting x = 5 into one of the equations:
y = 10 - x
y = 10 - 5
y = 5

So the answer is C