Answer:
a rational expression is an expression that has a RATIO in it
look at the word "RATIO"nal
a ratio is just a fraction...
so your question is "how to simplify fractions"
lots of approaches, this is one of them...
PRIME FACTOR the numerator and denominator
90 = 2 * 3 * 3 * 5
135 = 3 * 3 * 3 * 5
= 2/3 (the two 3's cancel and the 5's cancel)
Explanation: