Rational numbers are anything that can be expressed in the form a/b (that is, as a fraction aka a ratio). Pi is a value that goes on forever, so you can't set that up as a ratio; it's a famous irrational number. The square root of 34 is not even. If you put it in a calculator it gives you an ugly decimal, and if you try to convert it to a fraction, you can't. The same can be said for the square root of 40. The square root of 49, if you evaluate it, however, does give you a result you can work with: positive or negative 7. Both of these can be expressed as a ratio like 7/1 or -7/1.
