原式=(1-1/3)+(1/3 - 1/5)+(1/5-1/7)+(1/7-1/9)+(1/9-1/11)
= 1-1/3+1/3-1/5+1/5-1/7+1/7-1/9+1/9-1/11
= 1-1/11
= 10/11