Question

If the two vectors are equal in magnitude and resultant is doubled than angle between two vectors is
A 0°
B 60°
C 90°
D 180°

Answer 1

R²=A²+B²−2AB(cosβ)

But the magnitude of each vector is the same, therefore

A²=A²+A²−2(A)(A)(cosβ)

cosβ=−A²/−2(A²)

β=60

θ=180−60=120

Answer 2

The angle between vector A and vector B is θ as shown above.

The resultant is shown in red. Using the cosine rule:

R2=A2+B2−2AB(cosβ)

But the magnitude of each vector is the same, therefore

A2=A2+A2−2(A)(A)(cosβ)

cosβ=−A2−2(A2)

β=60∘

θ=180∘−60∘=120∘