A={x|x=√2m+n,m,n∈Z},∵x1∈A,x2∈A∴x1=√2m1+n1,x2=√2m2+n2m1,n1,m2,n2∈Z∴x1x2=(√2m1+n1)(√2m2+n2)=2m1m2+√2(m1n2+m2n1)+n1n2=√2(m1n1+m2n2)+(2m1m2+n1n2)∵m1n1+m2n2∈Z,2m1m2+n1n2∈Z∴x1x2∈A...m1∈Zm2∈Z? 为什么？题目中不是只列出了n∈Z？m,n∈Z我明白了，谢谢您的指导ok