1/(2n+1)(2n+3)=1/2[1/(2n+1) - 1/(2n+3)]
1/3*5+1/5*7+.+1/1997*1999
=1/2(1/3-1/5+1/5-1/7+……1/1997-1/1999)
=1/2(1/3-1/1999)
=998/5997
1/(2n+1)(2n+3)=1/2[1/(2n+1) - 1/(2n+3)]
1/3*5+1/5*7+.+1/1997*1999
=1/2(1/3-1/5+1/5-1/7+……1/1997-1/1999)
=1/2(1/3-1/1999)
=998/5997