x1,x2,x3的平均数是a所以(x1+x2+x3)/3=ax1+x2+x3=3a3x1,3x2,3x3的平均数是(3x1+3x2+3x3)/3=3(x1+x2+x3)/3=3ax1-1,x2-1,x3-1的平均数是[(x1-1)+(x2-1)+(x3-1)]/3=[(x1+x2+x3)-3]/3=(3a-3)/3=a-1