Consider the Fig 4.5 that shows a four bar mechanism,

Here,

Velocity at point B, V_B = ₁. AB = ₀. BO

Or,

₀ = ₁. AB/ BO

Velocity at point C, V_C = ₂. CD = ₀. CO

Or,

₀ = ₂. CD/ CO

Using the above two equations, we get the following ratio-

₁/ ₂ = CD/ AB . BO/ CO