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A cylinder of radius R is surrounded by a cylindrical shell of inner radius R and outer radius 2R. The thermal conductivity of the material of the inner cylinder is K 1 and the of the outer cylinder is K 2 . Assuming no loss of heat, the effective thermal conductivity of the system for heat flowing along the length of the cylinder is :

1. K 1 + K 2
2. 4 K 3 K 2 1 +
3. 2 K K 2 1 +
4. 5 K 3 K 2 2 1 +

Correct answer: 4 K 3 K 2 1 +

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