Q 11-03-018NEETJEE MainMedium
Two vectors $\vec{A}$ and $\vec{B}$ have equal magnitudes. If $|\vec{A} + \vec{B}| = \sqrt{3}\,|\vec{A} - \vec{B}|$, the angle between them is
Answer: (C) $60°$
Concept. For equal magnitudes $A$: $|\vec A \pm \vec B|^2 = 2A^2(1 \pm \cos\theta)$.
Step 1. Equation. $1 + \cos\theta = 3(1 - \cos\theta)$.
Step 2. Solve. $\cos\theta = \dfrac12 \Rightarrow \theta = 60°$.
Answer. $60°$ (Option C).
Solution by Sreeraj P, M.Sc Physics