18/08/15 01:55:12.46 HKfY+w2Q.net
>>953
x = (x-y) + y,
||ｘ||^2 = (x，x)
= (x-y，x-y) + (x-y，y) + (y，x-y) + (y，y)
= ||x-y||^2 + (x-y，y) + (y，x-y) + ||y||^2
≦ ||x-y||^2 + ||x-y||･||y|| + ||y||･||x-y|| + ||y||^2
= (||x-y|| + ||y||)^2,
||x|| - ||y|| ≦ ||x-y||,
xとyを入れ替えると
||y|| - ||x|| ≦ ||y-x|| = ||x-y||,