=1/2×（1-1/3+1/3-1/5+1/5-1/7+17-1/9+1/9-1/11）=1/2×（1-1/11）=1/2×10/11=5/11
1/(1×3)+1/(3×5)+1/(5×7)+1/(7×9)+1/(9×11)=(1/2)(1-1/3+1/3-1/5+1/5-1/7+1/7-1/9+1/9-1/11)=(1/2)(1-1/11)=(1/2)(10/11)=5/11
