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Subtracting Algebraic Fractions

(KS3, Year 8)

Algebraic fractions can be subtracted. Imagine you wanted to subtract ^a⁄_b and ^c⁄_d. a over b minus c over d

How to Subtract Algebraic Fractions

To subtract algebraic fractions, use the rule: a over b minus c over d equals a d minus b c over c d

A Real Example of How to Subtract Algebraic Fractions

Question

Subtract the algebraic fractions below. x over 2 minus y over 3

Step-by-Step:

1

Compare the fractions you are subtracting with the rule shown above. compare a over b minus c over d and x over 2 minus y over 3

By comparing, we see that a = x, b = 2, c = y, d = 3.

2

Use the rule, with a = x, b = 2, c = y, d = 3: a d minus bc over bd, replacing a with x, b with 2, c with y and d with 3

3

Calculate the terms in the rule. Where we have written two numbers or letters in brackets together, multiply them together:

(x)(3) = x × 3 = 3x

(2)(y) = 2 × y = 2y

(2)(3) = 2 × 3 = 6

Answer:

We have subtracted ^x⁄₂ and ^y⁄₃: x over 2 minus y over 3 equals 3 x minus 2 y over 6

Lesson Slides

The slider below shows a real example of how to subtract algebraic fractions.

Understanding The Rule

a over b minus c over d equals a d minus b c over b d

The letters written next to each other denotes that they are multiplying each other. The rule works when the a, b, c and d are numbers, letters, terms or expressions. Make sure you can:

Why Does This Work?

When subtracting fractions (algebraic or not) all of the fractions must have a common denominator. If initially the denominators are not the same... a over b plus c over d

...multiplying the denominators together will make a common denominator. b times d equals b d

But having multiplied the denominator of each fraction, the numerator must be multiplied by the same value if we are not to change the fraction. multiply the numerator by the same value as the denominator