Wang Xiu Ying is gift wrapping boxes in the shape of a cube for her class fundraiser. She calculates that the surface area, S SS, in square feet, of a cube-shaped box with a side of m mm feet is given by the function S ( m ) = 6 m 2 S(m)=6m 2 S, left parenthesis, m, right parenthesis, equals, 6, m, squared. She also finds that the cost, C CC, in dollars, of wrapping a box with a surface area of x xx square feet is given by the function C ( x ) = 0.15 x + 0.25 C(x)=0.15x+0.25C, left parenthesis, x, right parenthesis, equals, 0, point, 15, x, plus, 0, point, 25. Find an explicit expression that models the costs of wrapping a cube shaped box with a side length of m mm feet.

Step-by-step explanation:

The surface area of a cube-shaped box with a side of m feet is given by the function:

The cost, C, in dollars, of wrapping a box with a surface area of x square feet is given by the function:

We want to find an explicit expression that models the costs of wrapping a cube-shaped box with a side length of m feet.

In the cost function, C(x), x is the surface area, therefore:

To find the cost of wrapping a cube of side length m feet, we use the explicit function:

  x = 4 ± (1/7)√721

Step-by-step explanation:

7x – 9 = 7x^2 – 49

-9 = 7x^2 -56x . . . . . . . . subtract 7x

-9 = 7(x^2 -8x) . . . . . . . . factor out leading coefficient

-9 +7(16) = 7(x^2 -8x +16) . . . . . add the square of (-8/2) inside parentheses

103 = 7(x -4)^2 . . . . . collect terms, write as a square

103/7 = (x -4)^2 . . . . divide by the leading coefficient

±√(103/7) = x -4 . . . . . take the square root

x = 4 ± (1/7)√721 . . . . add 4, rationalize the denominator

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You "complete the square" by making sure the constant in the quadratic is the square of half the x-term coefficient. If changes are made on one side of the equation, the same changes need to be made on the other side of the equation. Here, we added 7(16) to complete the square.