Two rods $A$ and $B$ of different materials are welded together as shown in  figure.Their thermal conductivities are $K_1$ and $K_2$  The thermal conductivity of the composite rod will be

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  • [NEET 2017]
  • A

    $\frac{{3{K_1}{K_2}}}{2}$

  • B

    $\frac{{{K_1} + {K_2}}}{3}$

  • C

    $\;\frac{{{K_1}{K_2}}}{{3({K_1} + {K_2})}}$

  • D

    $\;\frac{{{K_1} + {K_2}}}{2}$

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  • [IIT 2001]

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