M(1)==2M(2)-M(1)==4-2==2M(3)-M(2)==7-4==3M(4)-M(3)==11-7==4..............M(n)-M(n-1) ==n以上各式相加可得M(n)==2+(2+3+....+n)==1+(1+2+...n)==1+n(1+n)/2=n^2/2+n/2+1