According to defintion of rational number : a number of the form p/q such tha q is nonzero is a rational number
Intergers are . .......-3,-2,-1,0,1,2,3......
And we know there there is always 1 in the denominator of each of integer that is its like ....-3/1,-2/1 ,0/1 ,1/1,2/1 .....
so all integers are rationals