1/[a×(a+n)]=(1/n)[1/a+1/(a+n)]
此题中,n=2。
原式=(1/2)×(1/1-1/3)+(1/2)×(1/3-1/5)+(1/2)×(1/5-1/7)+···+(1/2)×(1/49-1/51)
=(1/2)×(1/1-1/3+1/3-1/5+1/5-···-1/49+1/49-1/51)
=(1/2)×[1-(1/3-1/3)-(1/5-1/5)-···-(1/49-1/49)-1/51]
=(1/2)×(1-0-0-···-0-1/51)
=(1/2)×(1-1/51)
=(1/2)×(50/51)
=25/51