Interactive Mathematics by Kunio Mitsuma, Ph.D.

Yours Truly answers:
Listen to my audio feed as you read the answer below.

When two fractions are added (or one fraction is subtracted from another), the first task is to create the least common denominator (LCD).

A common mistake made in this particular problem is to multiply y – 1 and 1 – y and say that (y – 1)(1 – y) is the LCD. This is not the LCD.

The trick is to factor –1 out of 1 – y and make it – (y – 1). Then, we will see that the LCD is actually y – 1.

There is no universal formula for dealing with addition and subtraction of rational expressions. But, obtaining the correct LCD is as powerful as having such a formula.