NDA 2018 Maths · Mathematics · Vector Algebra · easy

If |vec a|=|vec b|=|vec a+vec b| for non-zero vectors, the angle between vec a and vec b is

Single correct option

|\vec a|=|\vec b|=|\vec a+\vec b|

for non-zero vectors, the angle between

\vec a

and

\vec b

Show answer and solution

Answer: (C) $120^\circ$

Let each magnitude be

k

. Then

k^2=|\vec a+\vec b|^2=k^2+k^2+2k^2\cos\theta

, so

\cos\theta=-\frac12

and

\theta=120^\circ

. The answer

60^\circ

is the angle for

|\vec a-\vec b|=k

instead.

Original question written in the NDA 2018 Maths pattern and checked twice for the correct answer. Not an official question paper. How we write and check questions · Corrections policy · Report a mistake