A△B= B/(A*B)-A/(A*B) = 1/A-1/B
1△2+2△3+3△4+…+2004△2005 = 1/1-1/2+1/2-1/3+1/3-1/4+...+1/2004-1/2005
中间项可以全部消掉,剩下1/1-1/2005 = 2004/2005
A△B= B/(A*B)-A/(A*B) = 1/A-1/B
1△2+2△3+3△4+…+2004△2005 = 1/1-1/2+1/2-1/3+1/3-1/4+...+1/2004-1/2005
中间项可以全部消掉,剩下1/1-1/2005 = 2004/2005