Question

The resultant C of A and B is perpendicular to A also|a| |=| |c| the angle between A and B is
Solution

c=A+B
C.A=0
(A+B).A=0
A²+AB-cos(0)=0
(0 is L between A&B)
0=cos-1(-a/b)
[d²=|A|²+|b|²-2AB.cos0
|C|=|A| = 0= cos-1(b/2a)