设n=1/2m那么 a=n+1 b=n+2 c=n+3a的平方+2ab+b的平方—2ac+c的平方—2bc =（a+b)^2-c(2a-c+2b)=(2n+3)^2-(n+3)(3n+3)=4n^2+12n+9-(3n^2+12n+9)=n^2=(1/2m)^2=1/(4m^2)
