Theory of Machine
In a band and block brake, the ratio of tensions on tight side and slack side of the band is (where μ = Coefficient of friction between the blocks and the drum, θ = Semi-angle of each block subtending at the center of drum, and n = Number of blocks)

T₁/T₂ = μ. θ. n
T₁/T₂ = [(1 + μ tanθ)/(1 - μ tanθ)]n
T₁/T₂ = [(1 - μ tanθ)/(1 + μ tanθ)]n
T₁/T₂ = (μ θ)n

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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
None of the listed here
Vary with the configuration of the mechanism
Moves as the mechanism moves, but joints are of permanent nature

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