Applied Mechanics and Graphic Statics
Lami's theorem states that

Three forces acting at a point are always in equilibrium
Three coplanar forces acting at a point will be in equilibrium, if each force is proportional to the sine of the angle between the other two
If three forces acting on a point can be represented in magnitude and direction by the sides of a triangle, the point will be in the state of equilibrium
Three coplanar forces acting at a point will be in equilibrium if each force is inversely proportional to the sine of the angle between the other two

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Applied Mechanics and Graphic Statics
The angle of friction is:

The force of friction when the body is in motion
The angle between the normal reaction and the resultant of normal reaction and limiting friction
The ratio of the friction and the normal reaction
The force of friction at which the body is just about to move

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