②对任意的[a],[b],[c]∈Z5,有([a]+[b])+[c] = [a+b]+[c] = [a+b+c],[a]+([b]+[c]) = [a]+[b+c] = [a+b+c],所以([a]+[b])+[c] = [a]+([b]+[c]),因此适合结合律,即满足了群公理的第二条结合律;③当a<0时,[a] = [-1]+[-1]+...+[-1](-a个[-1]相加) = [-1]^(-a) = [1^(-1)]^(-a) = [1]^a.综上,对任意的[a]∈Z5,都有[a] = [1]^a。