[44] For any x,y ∈ R^n, a ∈ R, |x+y| ≦ |x| + |y| (the equality holds only when x and y are linearly dependent and (x|y) ≧ 0.)

https://www.evernote.com/shard/s29/sh/86a5b991-db49-43a5-b524-d246fc8c3af7/80bba97621801cda77158fdaeb1bc848