If f(x + y, x – y) = xy for all x, y ∈ R, then the value of f(5, 1) is

2 3 4 6

6

Let x + y = 5 and x – y = 1. Then x = 3 and y = 2 ∴ f(5, 1) = f(x + y, x – y) = xy = 3(2) = 6