Theory of Machine
In a disc clutch, if there are n₁ number of discs on the driving shaft and n₂ number of discs on the driven shaft, then the number of pairs of contact surfaces will be

n₁ + n₂ - 2
n₁ + n₂
n₁ + n₂ - 1
n₁ + n₂ + 1

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Theory of Machine
In a mechanism, the fixed instantaneous centers are those centers which

Remain in the same place for all configurations of the mechanism
Vary with the configuration of the mechanism
Moves as the mechanism moves, but joints are of permanent nature
None of the listed here

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