Applied Mechanics and Graphic Statics
A system of coplanar forces is in equilibrium when

All the forces are concurrent
Force polygon closes
Funicular polygon closes
Both force polygon" and funicular polygon close

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

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

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