Mathematics » Rational Expressions and Equations » Simplify Complex Rational Expressions

# Simplifying a Complex Rational Expression by Using the LCD

## Simplifying a Complex Rational Expression by Using the LCD

We “cleared” the fractions by multiplying by the LCD when we solved equations with fractions. We can use that strategy here to simplify complex rational expressions. We will multiply the numerator and denominator by LCD of all the rational expressions.

Let’s look at the complex rational expression we simplified one way in this example from the previous lesson. We will simplify it here by multiplying the numerator and denominator by the LCD. When we multiply by $$\frac{\text{LCD}}{\text{LCD}}$$ we are multiplying by 1, so the value stays the same.

## Example

Simplify: $$\cfrac{\frac{1}{3}+\frac{1}{6}}{\frac{1}{2}-\frac{1}{3}}.$$

### Solution  The LCD of all the fractions in the whole expression is 6. Clear the fractions by multiplying the numerator anddenominator by that LCD.  Distribute.  Simplify.      ## Example: How to Simplify a Complex Rational Expression by Using the LCD

Simplify: $$\cfrac{\frac{1}{x}+\frac{1}{y}}{\frac{x}{y}-\frac{y}{x}}.$$

### Solution   ### Simplify a complex rational expression by using the LCD.

1. Find the LCD of all fractions in the complex rational expression.
2. Multiply the numerator and denominator by the LCD.
3. Simplify the expression.

Be sure to start by factoring all the denominators so you can find the LCD.

## Example

Simplify: $$\cfrac{\frac{2}{x+6}}{\frac{4}{x-6}-\frac{4}{{x}^{2}-36}}.$$

### Solution  Find the LCD of all fractions in the complex rationalexpression. The LCD is $$\left(x+6\right)\left(x-6\right)$$. Multiply the numerator and denominator by the LCD.  Simplify the expression. Distribute in the denominator.  Simplify.  Simplify.  To simplify the denominator, distributeand combine like terms.  Remove common factors.  Simplify.  Notice that there are no more factorscommon to the numerator and denominator.

## Example

Simplify: $$\cfrac{\frac{4}{{m}^{2}-7m+12}}{\frac{3}{m-3}-\frac{2}{m-4}}.$$

### Solution  Find the LCD of all fractions in thecomplex rational expression. The LCD is $$\left(m-3\right)\left(m-4\right)$$. Multiply the numerator and denominator by the LCD.  Simplify.  Simplify.  Distribute.  Combine like terms.  ## Example

Simplify: $$\cfrac{\frac{y}{y+1}}{1+\frac{1}{y-1}}.$$

### Solution  Find the LCD of all fractions in the complexrational expression. The LCD is $$\left(y+1\right)\left(y-1\right)$$. Multiply the numerator and denominator by the LCD.  Distribute in the denominator and simplify.  Simplify.  Simplify the denominator, and leave the numeratorfactored.    Factor the denominator, and remove factors commonwith the numerator.  Simplify.  