证明\x+y\<\X\+\Y\
证明,因为|x+y|>=0,|x|>=0,|y|>=0 (|x|+|y|)^2-|x+y|^2=x^2+2|xy|+y^2-(x^2+2xy+y^2) =2|xy|-2xy>=0 所以|x|+|y|>=|x+y|