Example

Subtracting xx3x2x+3\frac{x}{x-3} - \frac{x-2}{x+3}

Subtract two rational expressions whose denominators are distinct linear binomials, demonstrating careful sign distribution when subtracting the numerators:

xx3x2x+3\frac{x}{x - 3} - \frac{x - 2}{x + 3}

Step 1 — Find the LCD and rewrite each fraction. The denominators (x3)(x - 3) and (x+3)(x + 3) are already fully factored and share no common factor, so the LCD is their product: (x3)(x+3)(x - 3)(x + 3). The first fraction is missing (x+3)(x + 3); the second is missing (x3)(x - 3). Multiply each by the appropriate form of 11:

x(x+3)(x3)(x+3)(x2)(x3)(x+3)(x3)\frac{x(x + 3)}{(x - 3)(x + 3)} - \frac{(x - 2)(x - 3)}{(x + 3)(x - 3)}

Keep the denominators in factored form. Distribute in each numerator: x(x+3)=x2+3xx(x + 3) = x^2 + 3x and (x2)(x3)=x25x+6(x - 2)(x - 3) = x^2 - 5x + 6:

x2+3x(x3)(x+3)x25x+6(x3)(x+3)\frac{x^2 + 3x}{(x - 3)(x + 3)} - \frac{x^2 - 5x + 6}{(x - 3)(x + 3)}

Step 2 — Subtract the rational expressions. Subtract the numerators and place the difference over the common denominator. Because the second numerator has multiple terms, enclose it in parentheses and distribute the negative sign to every term:

x2+3x(x25x+6)(x3)(x+3)\frac{x^2 + 3x - (x^2 - 5x + 6)}{(x - 3)(x + 3)}

Distribute: (x25x+6)=x2+5x6-(x^2 - 5x + 6) = -x^2 + 5x - 6. Combine like terms: x2x2=0x^2 - x^2 = 0, 3x+5x=8x3x + 5x = 8x, and 6-6 remains:

8x6(x3)(x+3)\frac{8x - 6}{(x - 3)(x + 3)}

Step 3 — Simplify, if possible. Factor the numerator: 8x6=2(4x3)8x - 6 = 2(4x - 3). Neither 22 nor (4x3)(4x - 3) matches any denominator factor, so the expression is already in simplified form:

2(4x3)(x3)(x+3)\frac{2(4x - 3)}{(x - 3)(x + 3)}

This example follows the same three-step procedure used for addition but requires extra care with signs during Step 2. Every term in the second numerator — including 5x-5x becoming +5x+5x and +6+6 becoming 6-6 — must have its sign changed when the negative is distributed. Mishandling these signs is the most frequent error when subtracting rational expressions with unlike denominators.

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Updated 2026-04-21

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