几道数学题.请大家来做做.1999又1997分之1*1998分之19971 1 1 1 1———+———+———+———

1999又1997分之1*1998分之1997
=(1999+1/1997)*1997/1998
=1999*1997/1998+1/1997*1997/1998
=[1998+1]*1997/1998+1/1998
=1998*1997/1998+1997/1998+1/1998
=1997+(1997+1)/1998
=1997+1
=1998
1/3*4*5+1/4*5*6+.+1/98*99*100
=1/2[1/3*4-1/4*5]+1/2[1/4*5-1/5*6]+.+1/2[1/98*99-1/99*100]
=1/2[1/3*4-1/4*5+1/4*5-1/5*6+.+1/98*99-1/99*100]
=1/2[1/3*4-1/99*100]
=1/2[1/12-1/9900]
=1/2*206/2475
=103/2475