Thread: choosing lowest common denominator

How do I find the lcd from say x-4 and x+2 (those are the denominators). Is there any rule that always works?? Can¨'t figure out this by myself.. Thanks

Originally Posted by strunz

How do I find the lcd from say x-4 and x+2 (those are the denominators). Is there any rule that always works?? Can¨'t figure out this by myself.. Thanks

Hi strunz,

In your situation, the LCD is (x - 4)(x + 2). If you're having trouble with LCDs in general, see this lesson:


Adding and Subtracting Rational Expressions

I would say just multiple both of the denominators to get the lcd.

In general, you have to determine if the denominators have any common factors. For instance, the lowest common denominator in the example
is because the two fractions' denominators share the common factor of x + 3. If the denominators share no common factors, then the lowest common denominator is the product of each of the individual denominators.