let CNS be ComplexNormSpace; :: thesis: for x, y being Point of CNS holds ||.(x - y).|| <= ||.x.|| + ||.y.||
let x, y be Point of CNS; :: thesis: ||.(x - y).|| <= ||.x.|| + ||.y.||
||.(x - y).|| <= ||.x.|| + ||.(- y).|| by Def13;
hence ||.(x - y).|| <= ||.x.|| + ||.y.|| by Th103; :: thesis: verum