A = {1, 2, 3, 4, 5}
B = {1, 2, 3, 4 …}
f: A → B, f(x) = x2
∴ f(1) = 12 = 1
f(2) = 22 = 4
f(3) = 32 = 9
f(4) = 42 = 16
f(5) = 52 = 25
∴ Range of f = {1, 4, 9, 16, 25)
Elements in A have been different elements in B.
Therefore it is one-one function. But not all the elements in B have preimages in A. Therefore it is not on-to function.