1.x∈M,y∈M
则设X=m+n根号5,Y=p+q根号5(m,n,p,q属于Z)
则x+y=(m+n根号5)+(p+q根号5)
=(m+p)+(n+q)根号5
x-y=(m+n根号5)-(p+q根号5)
=(m-p)+(n-q)根号5
x*y=(m+n根号5)*(p+q根号5)
=mp+(np+mq)根号5+5nq
=(mp+5nq)+(np+mq)根号5
x/y=(m+n根号5)/(p+q根号5)
=[(m+n根号5)(p-q根号5)]/[(p+q根号5)(p-q根号5)]
=[(mp-5nq)+(np-mq)根号5]/[p^2-5q^2]
又m,n,p,q属于Z
则有:(m+p),(n+q),(m-p),(n-q),(mp+5nq),(np+mq)属于Z
则:x+y,x-y,xy属于M,
x/y不一定属于M
2.(1)若c属于C,
则c=6n+3
=(3n+1)+(3m+2)
=3(n+m)+3,n∈Z
∵3n+1∈A,3m+2∈B,
设a=3n+1,b=3m+2,
则c=a+b,结论成立.
(2)
不一定,a=3*1+1,b=3*2+2,a+b=12但不属于C