1993又1/2-1992又1/3+1991又1/2-1990又1/3+...+1又1/2-1/3
=1993又1/2+1991又1/2+.+1又1/2-(1992又1/3+1990又1/3+...+1/3)
=(1993+1991+...+1)+997X1/2-(1992+1990+.+2)-997X1/3
=(1993+1991+...+1)-(1992+1990+.+2)+997X1/2-997X1/3
=1993-1992+1991-1990+...+3-2+1+997/6
=996+1+997/6
=997+997/6
=7X997/6
=6979/6