, , , .,3.1.1,1, 2 ,1.,.,2.,a+b=b+a,(a+b)+c=a+(b+c),:,(ab)c=a(bc),:,(a+b)c=ac+bc,ab=ba,2.,a+b=b+a,1+2=2+1,1+2=2+1 12 ,2.,a+b=b+a,(a+b)+c=a+(b+c),:,(ab)c=a(bc),:,(a+b)c=ac+bc,ab=ba, ,2.,a+b=b+a,(a+b)+c=a+(b+c),:,(ab)c=a(bc),:,(a+b)c=ac+bc,ab=ba,abc ,2.,a+b=b+a,(a+b)+c=a+(b+c),:,(ab)c=a(bc),:,(a+b)c=ac+bc,ab=ba, ,1a_,2ab_,100a,ab,档,3a,b5 _ ,10a+5b,( ),(4)s.10_,s,10,5 1 ,m,򹲻 _,3,1,31500t ,1axbabab,2 棬5nn5,4. 磨2a+bkm 2a+b km,1_,2_,3_,4_,5nn 100 2009,4,7,10,13,n,n,4+3(n-1)=3n+1,n-1,n,1+3n,n,n,n+n+(n+1)=3n+1,n,n,n, 塣, 3.112, A=X+Y+Z,A X Y Z,