The resultant of two forces one double the other in magnitude is perpendicular to the smaller of the two forces. the angle b/w the two forces is? a. 120 b/ 60 c/ 90

I wold say a. 120°.

Considering the vector diagram: You can see that the resultant ##vecR## is perpendicular to the smaller vector ##vecA##. The angle ##alpha## can be found from trigonometry considering the triangle of hypotenuse ##2## and opposite side ##1## and get: ##1=2sin(alpha)## so that ##sin(alpha)=1/2## and ##alpha=30°## giving a total angle of: ##90°+30°=120°##