In a composite rod, when two rods of different lengths $l_1$ and $l_2$ and of the same cross-sectional area are joined from end to end then if $K$ is the effective coefficient of thermal conductivity, the value of $(l_1 + l_2)/K$ is
In a composite rod, when two rods of different lengths $l_1$ and $l_2$ and of the same cross-sectional area are joined from end to end then if $K$ is the effective coefficient of thermal conductivity, the value of $(l_1 + l_2)/K$ is
- A
$\frac{{{l_1}}}{{{K_1}}} + \frac{{{l_2}}}{{{K_2}}}$
- B
$\frac{{{l_1}}}{{{K_2}}} + \frac{{{l_2}}}{{{K_1}}}$
- C
$\frac{{{l_1}}}{{{K_1}}} - \frac{{{l_2}}}{{{K_2}}}$
- D
$\frac{{{l_1}}}{{{K_2}}} - \frac{{{l_2}}}{{{K_1}}}$
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