Question

(09.02 HC)

The function f(x) = −x2 + 24x − 80 models the hourly profit, in dollars, a shop makes for selling coffee, where x is the number of cups of coffee sold, and f(x) is the amount of profit.


Part A: Determine the vertex. What does this calculation mean in the context of the problem? (5 points)


Part B: Determine the x-intercepts. What do these values mean in the context of the problem? (5 points)


(10 points)

Answer 1

Vertex = (12,64)

The meaning of this pair of values is that the y-coordinate is the maximum profit obtainable, and the x-coordinate is how many cups sold will make the maximum profit.

X-intercepts: 4 and 20

The meaning of theses values is that these amounts of cups sold (4 and 20) will make zero profit.

Explanation:

To find the vertex we can use the formula for the x-coordinate of the vertex:

x_v = -b/2a

Where a and b are coefficients of the quadratic equation (in this case, a = -1 and b = 24)

So we have that:

x_v = -24 / (-2) = 12

The vertex is 12 cups of coffee. Now we apply this value to find the y-coordinate of the vertex:

f(x) = -12^2 + 24*12 - 80 = 64

So the vertex is (12,64). The meaning of this pair of values is that the y-coordinate is the maximum profit obtainable, and the x-coordinate is how many cups sold will make the maximum profit.

To find the x-intercepts, we need to make f(x) = 0 and find the values of x:

-x2 + 24x - 80 = 0

Delta = 24^2 - 80*4 = 256

sqrt(Delta) = 16

x1 = (-24 + 16)/(-2) = 4

x2 = (-24 - 16)/(-2) = 20

The x-intercepts are 4 and 20. The meaning of theses values is that these amounts of cups sold (4 and 20) will make zero profit.