An object is placed in front of a small plane mirror M1 and a large convex mirror M2 of focal length f .The distance between O and M1 is x and the distance between M1 and M2 is y .The images of 'O' formed by M1 and M2 coincide . The magnitude of 'f' is
1) x-y                 $2) \frac{x^2-y^2}{2y} 3) \frac{x^2+y^2}{2y} 4) \frac{x^2+y^2}{x+y}$