Let f(x)=ax2+bx+c, where a,b,c are rational, and f:Z→Z, where Z is the set of integers. Then a+b is
f:Z→Z is defined as f(x)=ax2+bx+c
which implies for integer inputs, the function gives integer outputs.
⇒f(0)=c=Z1 ...(1) (where Z1 is some integer)
Similarly, f(1)=a+b+c=Z2 ...(2) (where Z2 is some integer)
(2) - (1) gives a+b=Z2−Z1, which is also an integer.