Two rods whose lengths are l1 and l2 with heat conductivity coefficients k1 and k2 are placed end to end. The heat conductivity coefficient of a uniform rod of length l1 + l2 whose conductivity is same as that of the system of these two rods is
1
\(\frac{\left(l_{1}+l_{2}\right) k_{1} k_{2}}{k_{2} l_{1}+k_{1} l_{2}}\)
2
\(\frac{\left(l_{1}+l_{2}\right) k_{1} k_{2}}{k_{1} l_{1}+k_{2} l_{2}}\)
3
\(\frac{k_{1} l_{1}+k_{2} l_{2}}{\left(l_{1}+l_{2}\right) k_{1} k_{2}}\)
4
\(\frac{k_{1} l_{2}+k_{2} l_{1}}{\left(l_{1}+l_{2}\right) k_{1} k_{2}}\)