Numbers Finder by Sum and Reciprocal Sum

Input the sum and reciprocal sum of two numbers to quickly calculate the numbers.

Calculate Two Numbers Using Their Sum and Reciprocal Sum

Result

How to Calculate Two Numbers Using Their Sum and Reciprocal Sum

Let \( x \) and \( y \) be the two numbers, and assume their sum \( S \) and reciprocal sum \( R \) are known. The initial formulas are as follows: Sum of two numbers: \( x + y = S \) Reciprocal sum: \(\frac{1}{x} + \frac{1}{y} = R\)

  1. Expand the Reciprocal Sum Formula: \( \frac{1}{x} + \frac{1}{y} = \frac{x + y}{xy} \) Therefore: \( R = \frac{S}{xy} \)
  2. Solve for \( xy \): \( xy = \frac{S}{R} \)
  3. Using the sum-product relation, we construct the equation: \( t^2 - (x + y)t + xy = 0 \) Substituting \( x + y = S \) and \( xy = \frac{S}{R} \), the equation becomes: \( t^2 - St + \frac{S}{R} = 0 \)
  4. Using the quadratic formula: \( t = \frac{S \pm \sqrt{S^2 - 4 \cdot \frac{S}{R}}}{2} \) This gives the values of \( x \) and \( y \).

Examples

Example 1: Find two numbers whose sum is 8 and reciprocal sum is \( \frac{8}{15} \).

Solution:

1. Calculate \( xy \):

\( xy = \frac{S}{R} = \frac{8}{\frac{8}{15}} = 15 \)

2. Form the quadratic equation:

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

3. Solve for \( x \) and \( y \):

\( t = \frac{8 \pm \sqrt{8^2 - 4 \times 15}}{2} = \frac{8 \pm \sqrt{64 - 60}}{2} = \frac{8 \pm 2}{2} \)

\(x = 5\) and \(y = 3\)

Result: The two numbers are 5 and 3.

Example 2: Find two numbers whose sum is 11 and reciprocal sum is \( \frac{11}{28} \).

Solution:

1. Calculate \( xy \):

\( xy = \frac{S}{R} = \frac{11}{\frac{11}{28}} = 28 \)

2. Form the quadratic equation:

\( t^2 - 11t + 28 = 0 \)

3. Solve for \( x \) and \( y \):

\( t = \frac{11 \pm \sqrt{11^2 - 4 \times 28}}{2} = \frac{11 \pm \sqrt{121 - 112}}{2} = \frac{11 \pm 3}{2} \)

\(x = 7\) and \(y = 4\)

Result: The two numbers are 7 and 4.

Example 3: Find two numbers whose sum is 15 and reciprocal sum is \( \frac{3}{10} \).

Solution:

1. Calculate \( xy \):

\( xy = \frac{S}{R} = \frac{15}{\frac{3}{10}} = 50 \)

2. Form the quadratic equation:

\( t^2 - 15t + 50 = 0 \)

3. Solve for \( x \) and \( y \):

\( t = \frac{15 \pm \sqrt{15^2 - 4 \times 50}}{2} = \frac{15 \pm \sqrt{225 - 200}}{2} = \frac{15 \pm 5}{2} \)

\(x = 10\) and \(y = 5\)

Result: The two numbers are 10 and 5.