Question

Question #592c3

Answer 1

When the angle between them is between `90\deg and 270\deg`

Explanation:

scalar product of two vectors is given by `\vec{a}\dot\vec{b}=|a||b|cos\theta`,where `\theta` is the angle between the two vectors. The modulus of vectors are always positive, so the only way the scalar product is negative would be if `\cos \theta` is negative, which is for `(90, 270)\deg`