2X(1/1*3+1/3*5+1/5*7+1/7*9+1/9*11)=10
1/1*3=1/1-1/3 1/3*5=1/3-1/5 1/5*7=1/5-1/7 以下同理可得.所以原式等于
2X[(1/1)-(1/3)+(1/3)-(1/5)+(1/5)-(1/7)+(1/7)-(1/9)+(1/9)-(1/11)]=10
2X[1-(1/11)]=10
2X*(10/11)=10
2X=11
X=11/2