Here's the question you clicked on:
claireshowers
Using complete sentences, explain why it is necessary to have common denominators when adding or subtracting rational expressions.
In Arithmetic, we learned that fractions can be added or subtracted if and only if they have the same denominator. Similarly, rational expressions can be added or subtracted if and only if they have the same denominator. Thus, to add or subtract two rational expressions with unlike denominators, we must rewrite them as expressions with a common denominator. http://www.sparknotes.com/math/algebra2/rationalexpressions/section2.rhtml