设A=3m+1 B=3n+2 (m,n∈Z)
A-b=3m+1-3n-2=3(m-n)-1=3(m-n-1)+2
m-n-1∈Z,所以A-B∈N
AB=(3m+1)(3n+2)=9mn+3n+6m+2=3(mn+n+2m)+2
mn+n+2m∈Z所以AB∈N
设A=3m+1 B=3n+2 (m,n∈Z)
A-b=3m+1-3n-2=3(m-n)-1=3(m-n-1)+2
m-n-1∈Z,所以A-B∈N
AB=(3m+1)(3n+2)=9mn+3n+6m+2=3(mn+n+2m)+2
mn+n+2m∈Z所以AB∈N