Numbers Finder by Difference and Difference of Cubes

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Calculate Two Numbers Using Their Difference and Difference of Cubes

Result

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

Assume the two numbers are \( x \) and \( y \), with the following given:

  • Difference: \( x - y = D \)
  • Difference of Cubes: \( x^3 - y^3 = \Delta C \)

Step-by-Step Process:

  1. Set up relationships: \( x - y = D, \quad x^3 - y^3 = \Delta C \)
  2. Using the cubic difference formula: \( (x - y)^3 = x^3 - 3x^2y + 3xy^2 - y^3 \) Simplified formula: \( (x - y)^3 = x^3 - y^3 - 3xy(x - y) \)
  3. Substitute \( x - y = D \) and \( x^3 - y^3 = \Delta C \): \( D^3 = \Delta C - 3xy \cdot D \) Solve for \( xy \): \( xy = \frac{\Delta C - D^3 }{3D} \)
  4. Set up a quadratic equation and using the difference and product formulas for roots, represent \( x \) and \( y \): \( y^2 + (x - y)y - xy = 0 \)
  5. Solve for \( y \) using the quadratic formula: \( y = \frac{-D \pm \sqrt{D^2 + 4 \cdot xy}}{2} \)
  6. Find \( x \): \( x = y + D \)

Examples

Example 1: Two numbers have a difference of 5, and their difference of cubes is 335. What are the numbers?

Solution:

1. Calculate \( xy \):

\( xy = \frac{335 - 5^3}{3 \cdot 5} = 14 \)

2. Set up the quadratic equation:

\( y^2 + 5y - 14 = 0 \)

3. Calculate the discriminant:

\( \sqrt{5^2 + 4 \cdot 14} = \sqrt{25 + 56} = \sqrt{81} = 9 \)

4. Solve for \( y \):

\( y_1 = \frac{-5 + 9}{2} = 2 \)

\( y_2 = \frac{-5 - 9}{2} = -7 \)

5. Calculate \( x \):

\( x_1 = 2 + 5 = 7 \)

\( x_2 = -7 + 5 = -2 \)

Result: The numbers are (7, 2) or (-2, -7).

Example 2: Two numbers have a difference of 4, and their difference of cubes is 316. What are the numbers?

Solution:

1. Calculate \( xy \):

\( xy = \frac{316 - 4^3}{3 \cdot 4} = 21 \)

2. Set up the quadratic equation:

\( y^2 + 4y - 21 = 0 \)

3. Calculate the discriminant:

\( \sqrt{4^2 + 4 \cdot 21} = \sqrt{16 + 84} = \sqrt{100} = 10 \)

4. Solve for \( y \):

\( y_1 = \frac{-4 + 10}{2} = 3 \)

\( y_2 = \frac{-4 - 10}{2} = -7 \)

5. Calculate \( x \):

\( x_1 = 3 + 4 = 7 \)

\( x_2 = -7 + 4 = -3 \)

Result: The numbers are (7, 3) or (-3, -7).

Example 3: Two numbers have a difference of 3, and their difference of cubes is 999. What are the numbers?

Solution:

1. Calculate \( xy \):

\( xy = \frac{999 - 3^3}{3 \cdot 3} = 108 \)

2. Set up the quadratic equation:

\( y^2 + 3y - 108 = 0 \)

3. Calculate the discriminant:

\( \sqrt{3^2 + 4 \cdot 108} = \sqrt{9 + 432} = \sqrt{441} = 21 \)

4. Solve for \( y \):

\( y_1 = \frac{-3 + 21}{2} = 9 \)

\( y_2 = \frac{-3 - 21}{2} = -12 \)

5. Calculate \( x \):

\( x_1 = 9 + 3 = 12 \)

\( x_2 = -12 + 3 = -9 \)

Result: The numbers are (12, 9) or (-9, -12).