Two rods whose lengths are `l_1` and `l_2` and heat conductivity coefficient `x_1` and `x_2` are placed end to end. Find the heat conductivity coefficient of a uniform rod of length `l_1 + l_2` whose conductivity is the same as that of the system of these two rods. The lateral surfaces of the rods are assumed to be thermally insulated.

Answer

`lamda=(l_(1)+l_(2))/(l_(1)//lamda_(1)+l_(2)//lamda_(2))`