Numbers Finder by Sum and Sum of Cubes

Input the sum and sum of cubes to quickly find the two numbers.

Calculate Two Numbers Using Their Sum and Sum of Cubes

Result

How to Calculate Two Numbers Using Their Sum and Sum of Cubes

Given:

  • \( S \): the sum of two numbers (\( x + y \))
  • \( C \): the sum of their cubes (\( x^3 + y^3 \))

Steps:

  1. Set up relationships: \( x + y = S, \quad x^3 + y^3 = C \)
  2. Use the sum of cubes formula: \( (x + y)^3 = x^3 + 3x^2y + 3xy^2 + y^3 \)
  3. Simplified formula: \( (x + y)^3 = x^3 + y^3 + 3xy(x + y) \)
  4. Substituting the known values: \( S^3 = C + 3xy \cdot S \) \( xy = \frac{S^3 - C}{3S} \)
  5. Use the quadratic equation (based on sum and product) to find \( x \) and \( y \): \( t^2 - (x + y)t + xy = 0 \)
  6. Solve the quadratic equation: \( x, y = \frac{S \pm \sqrt{S^2 - 4 \cdot xy}}{2} \)

Examples

Example 1: Two numbers have a sum of 11, and the sum of their cubes is 737. What are the numbers?

Solution:

1. Calculate \( xy \):

\( xy = \frac{11^3 - 737}{3 \cdot 11} = 18 \)

2. Form the quadratic equation:

\( t^2 - 11t + 18 = 0 \)

3. Calculate the discriminant:

\( \sqrt{11^2 - 4 \cdot 18} = \sqrt{121 - 72} = \sqrt{49} = 7 \)

4. Solve for \( t \):

\( t = \frac{11 \pm 7}{2} \implies t = 9 \text{ or } 2 \)

Result: The numbers are 9 and 2.

Example 2: Two numbers have a sum of 9, and the sum of their cubes is 189. What are the numbers?

Solution:

1. Calculate \( xy \):

\( xy = \frac{9^3 - 189}{3 \cdot 9} = 20 \)

2. Form the quadratic equation:

\( t^2 - 9t + 20 = 0 \)

3. Calculate the discriminant:

\( \sqrt{9^2 - 4 \cdot 20} = \sqrt{81 - 80} = \sqrt{1} = 1 \)

4. Solve for \( t \):

\( t = \frac{9 \pm 1}{2} \implies t = 5 \text{ or } 4 \)

Result: The numbers are 5 and 4.

Example 3: The sum of two numbers is 8, and the sum of their cubes is 152. What are the numbers?

Solution:

1. Calculate \( xy \):

\( xy = \frac{8^3 - 152}{3 \cdot 8} = 15 \)

2. Form the quadratic equation:

\( t^2 - 8t + 15 = 0 \)

3. Calculate the discriminant:

\( \sqrt{8^2 - 4 \cdot 15} = \sqrt{64 - 60} = \sqrt{4} = 2 \)

4. Solve for \( t \):

\( t = \frac{8 \pm 2}{2} \implies t = 5 \text{ or } 3 \)

Result: The numbers are 5 and 3.