The magnitude of the resultant of two equal vectors is equal to the magnitude of either vector. What is the angle betwee the two vectors?
We know that, R2=A2+B2+2AB cos θ
It is given that R = A = B.
Putting these values we have
A2=A2+A2+2A2 cos θ
⇒cos θ=−12 which gives θ=120∘
Hence, the correct choice is (c).