How can we prove that the forces acting on a body can form a closed-up polygon?
If they didn't, then there would be a resultant force. The magnitude of the resultant is zero only if the other forces form a closed polygon. The condition for equilibrium is, of course, no net force
If there are no forces or only two forces on the object then that object could be in equilibrium but the force vectors can't be connected to form a polygon.
Well you could think of the two forces case as going along a line and back along the same line.
The idea to remember is that the sum of the force vectors is a zero vector for the object in equilibrium.
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