When three forces are acting at a point and they are in equilibrium, then each force is directly of proportional to the sine of the angle between the other two forces.
Let us consider A, B and C be three forces acting at a point O. as these three forces are in equilibrium, the point is at rest. Let α, β and γ be the angles opposite to the forces A, B and C respectively.
So, as per Lami’s theorem