let CNS be ComplexNormSpace; :: thesis: for x, y being Point of holds ||.(x - y).|| <= ||.x.|| + ||.y.||
let x, y be Point of ; :: thesis: ||.(x - y).|| <= ||.x.|| + ||.y.||
||.(x - y).|| <= ||.x.|| + ||.(- y).|| by Def11;
hence ||.(x - y).|| <= ||.x.|| + ||.y.|| by Th104; :: thesis: verum