已知集合M={x|x=3n,n∈Z},N={x|x=3n+1,n∈Z},P={x|x=3n-1,n∈Z},且a∈M,b∈N,c∈P,设d=a-b+c,则A.d∈M B.d∈N C.d∈P D以上都不对答案是a=3n b=3k+1 c=3m-1 d=3n-3k+3m-2 =3n-3k+3m-3+1= 3(n-k+m-1)+1但我唔明既然设了a=3n,点解唔设b系3n+1,C系3n-1
问题描述：
已知集合M={x|x=3n,n∈Z},N={x|x=3n+1,n∈Z},P={x|x=3n-1,n∈Z},且a∈M,b∈N,c∈P,设d=a-b+c,则
A.d∈M B.d∈N C.d∈P D以上都不对
答案是a=3n b=3k+1 c=3m-1
d=3n-3k+3m-2 =3n-3k+3m-3+1= 3(n-k+m-1)+1
但我唔明既然设了a=3n,点解唔设b系3n+1,C系3n-1
答