Example

Subtracting 4x243x2x2\frac{4}{x^2-4} - \frac{3}{x^2-x-2}

Subtract two rational expressions where finding the LCD involves factoring a difference of squares and a trinomial:

4x243x2x2\frac{4}{x^2 - 4} - \frac{3}{x^2 - x - 2}

Step 1 — Factor the denominators to find the LCD. Factor the first denominator as a difference of squares: x24=(x2)(x+2)x^2 - 4 = (x - 2)(x + 2). Factor the second denominator: x2x2=(x2)(x+1)x^2 - x - 2 = (x - 2)(x + 1). The LCD is (x2)(x+2)(x+1)(x - 2)(x + 2)(x + 1).

Step 2 — Rewrite each fraction with the LCD. Multiply each fraction by its missing factor: 4(x+1)(x2)(x+2)(x+1)3(x+2)(x2)(x+1)(x+2)\frac{4(x + 1)}{(x - 2)(x + 2)(x + 1)} - \frac{3(x + 2)}{(x - 2)(x + 1)(x + 2)}

Distribute the numerators: 4x+4(x2)(x+2)(x+1)3x+6(x2)(x+2)(x+1)\frac{4x + 4}{(x - 2)(x + 2)(x + 1)} - \frac{3x + 6}{(x - 2)(x + 2)(x + 1)}

Step 3 — Subtract the numerators. Distribute the negative sign across the second numerator: 4x+4(3x+6)(x2)(x+2)(x+1)\frac{4x + 4 - (3x + 6)}{(x - 2)(x + 2)(x + 1)}

4x+43x6(x2)(x+2)(x+1)\frac{4x + 4 - 3x - 6}{(x - 2)(x + 2)(x + 1)}

Combine like terms: x2(x2)(x+2)(x+1)\frac{x - 2}{(x - 2)(x + 2)(x + 1)}

Step 4 — Simplify. Cancel the shared factor (x2)(x - 2): 1(x+2)(x+1)\frac{1}{(x + 2)(x + 1)}

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Updated 2026-04-30

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