If A={1,2,3,4} and R={(a,b): a,b∈A, a+2=b}, what is the domain of R?

Correct answer: {1,2}

The governing concept is the domain of a relation. The domain consists of all first components a for which there exists a second component b in A satisfying a+2=b. Test the possible elements of A: if a=1, then b=3, which belongs to A, so (1,3) is valid. If a=2, then b=4, also in A, so (2,4) is valid. If a=3, then b=5, which is outside A; if a=4, then b=6, also outside A. Thus the first components that actually occur are 1 and 2, giving domain(R)={1,2}. Option A is correct. Option B is the range, while C includes first components that produce invalid second components.