1, there is a conclusion in mechanics: if the same object is in equilibrium under the action of only three forces, then these three forces must be on the same plane and the vector sum of these three forces is equal to zero, that is, these three forces can be connected end to end to form a closed triangle.

As shown in the figure, OA, OB and OC are three balancing forces. △OAD is obtained after translation, where vector DO=OB=F2, vector AD=OC=F3, and vector OA = f1; D, O and B are on the same straight line, and OADC is a parallelogram.

2. According to the sine theorem OA/sinADO=DO/sinOAD=AD/sinDOA,

Note that ∠ ADO = ∠ Doc = 180-∠ BOC, so OA/sinado = f1/sin (180-∠ b0c) = f1/.

Similarly, do/sinoad = F2/sin (180-∠ AOC) = F2/sinaoc;

AD/sin DOA = F3/sin( 180-∠AOB)= F3/sinAOB，

So there is f1/sinboc = f2/sinaoc = f3/sinaob.