一般问题应该是S=1/(1*3)+1/(2*4)+.+1/(49*51)
通项为 1/(n*(n+2)),解如下：
2＊S＝2/(1*3)+2/(2*4)+.+2/(49*51)
=(1/1-1/3)+(1/2-1/4)+(1/3-1/5)+(1/4-1/6)+.
..+(1/46-1/48)+(1/47-1/49)+(1/48-1/50)+(1/49-1/51)
=1/1+1/2-1/50-1/51
S=(1/1+1/2-1/50-1/51)/2