Definition
Rational number
A rational number can be written exactly as a fraction of two integers with a nonzero denominator.
What it means
A number is rational when two integers can build it as an exact fraction. Whole numbers count too: 5 = 5/1. Negative numbers can be rational as well.
Remember it
If an exact fraction of two integers can name it, the number is rational.
Example
−1.25 is rational because −1.25 = −125/100 = −5/4, and −5 and 4 are integers with a nonzero denominator.
Watch for
Numbers such as π are irrational: no fraction of two integers equals them exactly.
Common mix-up
Here “rational” means “can be written as a ratio”; it does not mean sensible or reasonable.
Exact meaning
A rational number is any number that can be written as a/b, where a and b are integers and b ≠ 0. Its decimal expansion terminates or eventually repeats.