State when the equality will hold, `|veca - vecb|ge|veca|-|vecb|`

Explanation

Let ABC be a triangle such that `vec(AB) = veca, vec(CB) = vecb`
Then `vec(AC) = veca-vecb` be triangle law In the `triangleABC, AC+CB ge AB`
`implies |veca-vecb|+|vecb| ge veca`
`implies |veca-vecb| ge |veca| - |vecb|`
therefore The equality holds when `veca` and `vecb` are collinear vectors.