x○y=xy-10(x+y)+110
(x○y)○z=(xy-10(x+y)+110)○z
=[xy-10(x+y)+110]z-10[
(xy-10(x+y)+110+z]+110
=xyz-10xz-10yz+110z-10xy+100x+100y-1100-10z+110
=xyz-10xz-10yz-10xy+100x+100y+100z-990-10z
x○(y○z)=x○[yz-10(y+z)+110]
=x[yz-10(y+z)+110]-10[x+yz-10(y+z)+110]+110
=xyz-10xy-10xz+110x-10x-10yz+100y+100z-1100+110
=xyz-10xz-10yz-10xy+100x+100y+100z-990-10z
(x○y)○z=x○(y○z)
