Introduction : specify coefficient, integers/fractions, rational number, constant.

Every fraction is a rational number.

Every integer is a rational number.

Introduction to Numbers

Every negative integer is a negative rational number.

Introduction to Rational Function

If f(x)=cos x+cos ax is periodic function, then a must be (a)an integer (b) a rational number (c)an irrational number (d) an even number

Every integer is a rational number but a rational number need not be an integer