Question

Under what condition is |A⃗ − B⃗ |=A+B?

Answer 1

ok i think you know |A⃗ − B⃗ | is magnitude of A⃗ − B⃗ so
\[ |A⃗ − B⃗ |=\sqrt{A^2 + B^2 - 2AB \cos \theta} \]
now put
\[\theta = \pi\]
you will get it.