1.1989×1999-1988×2000=(1988+1)×1999-1988×2000=1988×1999+1999-1988×2000=1999-1988×(2000-1999)=1999-1988=12.1/15+1/35+1/63+1/99+1/143+1/195=1/2(1/3-1/5)+1/2(1/5-1/7)+1/2(1/7-1/9)+1/2(1/9-1/11)+1/...