Math Problem Statement

The product of two positive numbers is 16. Determine these two numbers is that the square of one plus the cube of the other is the smallest.

Solution

We are tasked with finding two positive numbers whose product is 16, and such that the square of one number plus the cube of the other is minimized.

Let the two positive numbers be xx and yy, such that: xy=16x \cdot y = 16 We want to minimize the function: f(x,y)=x2+y3f(x, y) = x^2 + y^3 To simplify the problem, we can express yy in terms of xx using the product constraint xy=16x \cdot y = 16, which gives: y=16xy = \frac{16}{x} Now substitute this into the function to minimize: f(x)=x2+(16x)3=x2+4096x3f(x) = x^2 + \left(\frac{16}{x}\right)^3 = x^2 + \frac{4096}{x^3} Next, we find the critical points by differentiating f(x)f(x) with respect to xx and setting the derivative equal to zero. First, compute the derivative: f(x)=2x34096x4f'(x) = 2x - 3 \cdot \frac{4096}{x^4} Set f(x)=0f'(x) = 0 to find critical points: 2x=12288x42x = \frac{12288}{x^4} Multiply both sides by x4x^4 to clear the fraction: 2x5=122882x^5 = 12288 Solve for xx: x5=6144x^5 = 6144 x=61445x = \sqrt[5]{6144} Using the approximation 614455.2\sqrt[5]{6144} \approx 5.2.

Now, substitute x=5.2x = 5.2 into y=16xy = \frac{16}{x} to find yy: y=165.23.08y = \frac{16}{5.2} \approx 3.08

Thus, the two numbers are approximately x=5.2x = 5.2 and y=3.08y = 3.08.

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Here are 5 questions that relate to this problem:

  1. What would happen if the product constraint was different from 16?
  2. How do we find critical points using derivatives?
  3. What is the significance of minimizing the sum of squares and cubes in this context?
  4. Could this problem be solved using Lagrange multipliers?
  5. How does the approximation of 61445\sqrt[5]{6144} affect the accuracy of the result?

Tip: When minimizing or maximizing a function, always check the second derivative to confirm if the critical point is a minimum or maximum.

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Math Problem Analysis

Mathematical Concepts

Optimization
Algebra
Derivatives

Formulas

Product of two numbers: x * y = 16
Objective function: f(x, y) = x^2 + y^3
Substitution: y = 16 / x
Derivative of f(x): f'(x) = 2x - 3 * (4096 / x^4)

Theorems

Critical Points Theorem
Derivative Rules

Suitable Grade Level

Grades 10-12