(1998×1999)分1998+(1999×2000)分1998+(2000× 2001)分1998……+(2049×2050)分1998
=1998((1/1998-1/1999)+(1/1999-1/2000)+(1/2000-1/2001)+....+(1/2049-1/2050))
=1998(1/1998-1/2050)
=1-1998/2050
=52/2050
=26/1025
原式=1998(1/1998-1/1999)+1998(1/1999-1/2000).......+1998(1/2049-1/2050)
=1998(1/1998-1/1999+1/1999-1/2000.......+1/2049-1/2050)
=1998(1/1998-1/2050)
=1-1998/2050
=52/2050
=26/1025