At what angle must the two forces (x + y) and  (x - y) act, so that the resultant may be  x2+y2?

Let the two forces be A and B.Resultant,  R2=A2+B2+2ABcosθ                      …(i)Substituting, A=(x+y), B=(x-y)and     R=x2+y2On substituting these values in Eq. (i), we getθ=cos-1-x2+y22x2-y2