Example

Subtracting 8yy2164y4\frac{8y}{y^2-16} - \frac{4}{y-4}

Subtract two rational expressions where one denominator is a difference of squares that contains the other denominator as a factor:

8yy2164y4\frac{8y}{y^2 - 16} - \frac{4}{y - 4}

Step 1 — Find the LCD and rewrite each fraction. Factor the first denominator as a difference of squares: y216=(y4)(y+4)y^2 - 16 = (y - 4)(y + 4). The second denominator (y4)(y - 4) is already one of these factors. The LCD is (y4)(y+4)(y - 4)(y + 4).

The first fraction already has the LCD. The second fraction is missing the factor (y+4)(y + 4); multiply its numerator and denominator by (y+4)(y + 4):

8y(y4)(y+4)4(y+4)(y4)(y+4)\frac{8y}{(y - 4)(y + 4)} - \frac{4(y + 4)}{(y - 4)(y + 4)}

Distribute in the second numerator: 4(y+4)=4y+164(y + 4) = 4y + 16.

Step 2 — Subtract the rational expressions. Subtract the numerators over the common denominator, placing the second numerator in parentheses:

8y(4y+16)(y4)(y+4)\frac{8y - (4y + 16)}{(y - 4)(y + 4)}

Distribute the negative sign: 8y4y16=4y168y - 4y - 16 = 4y - 16:

4y16(y4)(y+4)\frac{4y - 16}{(y - 4)(y + 4)}

Step 3 — Simplify, if possible. Factor the numerator to look for common factors: 4y16=4(y4)4y - 16 = 4(y - 4). The factor (y4)(y - 4) appears in both the numerator and denominator — cancel it:

4(y4)(y4)(y+4)=4y+4\frac{4(y - 4)}{(y - 4)(y + 4)} = \frac{4}{y + 4}

This example illustrates a subtraction problem where the combined numerator shares a common binomial factor with the denominator, enabling significant simplification. Factoring y216y^2 - 16 as a difference of squares reveals that the second denominator (y4)(y - 4) is already contained within the first, so only the second fraction requires rewriting. After subtracting and factoring the numerator, the shared factor (y4)(y - 4) cancels, reducing the expression to a simple fraction.

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Updated 2026-04-30

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