Applied Mechanics and Graphic Statics
The characteristic of a couple is:

All listed here
Algebraic sum of moments of forces, constituting a couple, about any point, is same
A couple can be never the balanced by a single force
Algebraic sum of forces, constituting a couple is zero

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Applied Mechanics and Graphic Statics
The Law of Polygon of Forces states that

If forces acting on a point can be represented of a polygon taken in order, their sides of a polygon taken in order, their resultant will be represented in magnitude and direction by the closing side
If forces acting on a point can be represented in magnitude and direction by the sides of a polygon taken in order, then the resultant of the forces will be represented in magnitude and direction by t
If a polygon representing the forces acting at point in a body is closed, the forces are in equilibrium
If forces acting on a point can be represented in magnitude and direction by the sides of a polygon in order, the forces are in equilibrium

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