Magnitude of the resultant couple:
For non-parallel forces also the location of the line of action of resultant may be determined by utilizing R × d = ∑ M o .
The magnitude of the resultant couple is specified by
cos α x = ∑ Mx / M, cos α y =∑ My/ M, cos α z = ∑ Mz/ M
where ∑ M x , ∑ M y and ∑ M z are the algebraic sums of the moments of the couples of the system around x, y and z axes, respectively. The angles αx, αy and αz are the angles, the vector representing the couple M makes along with x, y and z axes, respectively.
In particular case of concurrent forces, M = 0.