Finding the angle opposite the R

[Drain Brain]

can you show me another way of finding the angle opposite the resultant

the way I find it is by constructing a parallelogram and knowing that the angles in a parallelogram add up to 360 deg and knowing that the angle between F1 and F2 is 25 deg then I subtracted twice of that to 360 deg getting 310 deg and dividing by 2 I get the angle opposite R as 155 deg.

can you show me another method on how to determine that without using parallelogram rule. thanks!

 

[MarkFL]

We may use the following:

View attachment 2820

$$a+25^{\circ}=180^{\circ}$$

$$a=155^{\circ}$$