(M/3-1/2)^2
=M^2/9-2*M/3*1/2+1/4
=M^2/9-M/3+1/4
原式=(M/3)^2--(1/2)^2--2X(M/3)(1/2)
=(M^2)/9--(1/4)--M/3
=(M^2--3M)/9--1/4