In the expressions below, is a nonzero rational number and is an irrational number.
Which expression must represent an irrational number?
Show answer & explanation
Answer: D
: Can be rational. If , then .
: Can be rational. If , then ; a product of two irrational numbers is not always irrational.
: Can be rational. If , then ; a sum of two irrational numbers is not always irrational.
: Correct. A nonzero rational number times an irrational number is always irrational: if were rational, then would be a quotient of rational numbers, which is rational.
Closure rules: rational ± rational and rational × rational stay rational; rational + irrational and nonzero rational × irrational are always irrational; two irrationals can combine to a rational.