If three forces acting at a point on a rigid body are in equilibrium, each force will be proportional to the sine of the angle between the other two forces. Refer Fig. 1.11 (a) & (b)
The mathematical expression of Lami’s theorem is p/sinα= Q /sin β =S /sin γ
Proof: Applying sine rule for the triangle (Fig. 1.11) (b)
p/ sin (180°- α)=Q sin (180°- β)= S / sin (180°- r)
p /sin α =Q sin β =S /sin γ. Proved.