"If a body is in equilibrium under the action of three forces, then each force is proportional to the sine of the angle between the other two forces." This statement is known as: 

Explanation:

• Lami's theorem is an equation relating the magnitudes of three coplanar, concurrent and non-collinear forces, which keeps an object in static equilibrium, with the angles directly opposite to the corresponding forces.
• According to the theorem:

Additional Information 

Varignon’s Principle of moments (or the law of moments):

It states, “If a number of coplanar forces are acting simultaneously on a particle, the algebraic sum of the moments of all the forces about any point is equal to the moment of their resultant force about the same point.”

i.e. R × r = F1 × r1 + F2 × r2