Theory of Machine
Lower pairs are those which have

Point or line contact between the two elements when in motion
Surface contact between the two elements when in motion
Two elements that permit relative motion
Elements of pairs not held together mechanically

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Theory of Machine
Two pulleys of radii r₁ and r₂ and at distance x apart are connected by means of an open belt drive. The length of the belt is

π (r₁ - r₂) + (r₁ + r₂)²/x + 2x
π (r₁ + r₂) + (r₁ + r₂)²/x + 2x
π (r₁ + r₂) + (r₁ - r₂)²/x + 2x
π (r₁ - r₂) + (r₁ - r₂)²/x + 2x

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