Let f and g be two functions given by f = {(2, 4), (5, 6), (8, −1), (10, −3)}

f = {(2, 4), (5, 6), (8, –1), (10, –3)}

g = {(2, 5), (7, 1), (8, 4), (10, 13), (11, –5)}

Domain of  f is {2, 5, 8, 10} and Domain of g = {2, 7, 8, 10, 11}

Since (f  + g) (x) = f(x) + g(x)

∴ x should lie in Both Domain of f and Domain of g

∴ x ∈ Domain ( f) ∩ Domain (g) 

i.e Domain of f  + g is {2, 8, 10}