The figure below shows a thin plano-convex lens of refractive index `mu_(1)` and a thin plano-concave lens of refractive index `mu_(2)`, both having same radius of curvature R of their curved surfaces. Another thin lens of refractive index `mu_(3)` has same radius of curvature R on the two surface between the plano-convex and plano-concae lenses that the plane surfaces are parallel to each other. Find the focal length of the combination.