Math Problem Statement

the sum of a number squared and a second number is 27. What are their values if you want to maximize their product

Solution

We are given the following information:

  1. The sum of the square of one number and a second number is 27. This translates to the equation: x2+y=27x^2 + y = 27 where xx is the first number and yy is the second number.

  2. We want to maximize the product of these two numbers, P=xyP = x \cdot y.

Step 1: Express yy in terms of xx

From the equation x2+y=27x^2 + y = 27, solve for yy: y=27x2y = 27 - x^2

Step 2: Write the product function PP

Now substitute y=27x2y = 27 - x^2 into the product function P=xyP = x \cdot y: P(x)=x(27x2)=27xx3P(x) = x \cdot (27 - x^2) = 27x - x^3 This is the function we want to maximize.

Step 3: Find the critical points by taking the derivative

To maximize P(x)P(x), we first find its derivative with respect to xx: dPdx=273x2\frac{dP}{dx} = 27 - 3x^2

Set the derivative equal to zero to find the critical points: 273x2=027 - 3x^2 = 0 3x2=273x^2 = 27 x2=9x^2 = 9 x=±3x = \pm 3

Step 4: Determine which critical point maximizes the product

We now check the second derivative to determine whether these points correspond to a maximum or minimum: d2Pdx2=6x\frac{d^2P}{dx^2} = -6x For x=3x = 3, we have: d2Pdx2=6(3)=18(which is negative, indicating a maximum)\frac{d^2P}{dx^2} = -6(3) = -18 \quad (\text{which is negative, indicating a maximum}) For x=3x = -3, we have: d2Pdx2=6(3)=18(which is positive, indicating a minimum)\frac{d^2P}{dx^2} = -6(-3) = 18 \quad (\text{which is positive, indicating a minimum})

Thus, the value x=3x = 3 gives a maximum.

Step 5: Find yy

When x=3x = 3, substitute into the equation x2+y=27x^2 + y = 27: 32+y=273^2 + y = 27 9+y=279 + y = 27 y=18y = 18

Conclusion

The values of the two numbers that maximize the product are x=3x = 3 and y=18y = 18. The maximum product is: P=318=54P = 3 \cdot 18 = 54

Do you need any further details or clarifications?

Here are some additional questions you might find interesting:

  1. How would the product change if we modified the sum condition to something different, like 30 instead of 27?
  2. Can the product be maximized for negative values of xx?
  3. What would the product be if we tried maximizing the sum of the two numbers instead of their product?
  4. How does the solution change if we restrict xx and yy to integer values only?
  5. How could we graph the function P(x)=27xx3P(x) = 27x - x^3 to visualize the maximum point?

Tip: Always check the second derivative to confirm whether a critical point corresponds to a maximum or a minimum.

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Math Problem Analysis

Mathematical Concepts

Algebra
Optimization
Derivative Calculus

Formulas

x^2 + y = 27
P = x * y
Derivative of P(x): dP/dx = 27 - 3x^2
Second derivative test: d^2P/dx^2 = -6x

Theorems

First derivative test for critical points
Second derivative test for concavity

Suitable Grade Level

Grades 11-12