1/1x3+1/3x5+1/5x7+...+1/1997x1999=?帮我啊!谢谢!

1/1x3+1/3x5+1/5x7+...+1/1997x1999=?帮我啊!谢谢!
1/1x3+1/3x5+1/5x7+...+1/1997x1999=?
帮我啊!谢谢!

1/1x3+1/3x5+1/5x7+...+1/1997x1999=?帮我啊!谢谢!
原式=1/2(1-1/3+1/3-1/5+...+1/1997-1/1999)=1/2(1-1/1999)=999/1999

1/[n*(n+2)]=1/2[1/n-1/(n+2)]
1/1x3+1/3x5+1/5x7+...+1/1997x1999
=0.5(1-1/3)+0.5(1/3-1/5)+0.5（1/5-1/7）+……+0.5(1/1997-1/1999)
=0.5(1-1/3+1/3-1/5+1/5-1/7+……+1/1997-1/1999)
=0.5(1-1/1999)
=0.8*1998/1999
=999/1999

题目里1x3、3X5、5X7、。。。。。要加括号，