A slab consists of two layers of two different materials of same thickness having thermal conductivities K1 and K2, the equivalent conductivity of the combination in series is

In series slabs are,for 2 slabs of same thickness l having coefficient of thermal conductivities K1,K2 is since, in series effective resistance is Rseries=R1+R2 Resistance is R=lKA, here l = thickness, K=coefficient of thermal conductivity, A=area of cross section Rseries=R1+R2 l+lKseriesA=lK1A+lK2A  ⇒Kseries=2K1K2K1+K2