(c) As we know that the magnitude of the resultant of two vectors X and Y, R2=X2+Y2+2XY‌cos‌θ where, θ is the angle between X and Y. Putting, X=(A+B) Y=(A−B) and ‌‌R=√A2+B2 in Eq. (i), we get A2+B2=(A+B)2+(A−B)2+2(A+B)(A−B)‌cos‌θ ⇒‌‌A2+B2=A2+B2+2AB+A2 +B2−2AB+2(A2−B2)‌cos‌θ ⇒‌