Terminating decimals are rational numbers. Infact, there is a formula to find the fraction form of them. Hence, a/b, the definition of rational numbers. Non terminating decimals might or might not be: if there's a pattern you can decipher and write in the form a/b then yes. Pi can't be expressed in the form a/b. There is a fraction I remember that is close to it (3.142) but that's not the same.