(2/(1*3))+(2/(3*5))+(2/(5*7))+...+(2/(9*11)).
(2/(1*3))+(2/(3*5))+(2/(5*7))+…+(2/(9*11))．
<gadget id="calculator">1 / 11</gadget>
<output>1/11 = around 0.090909</output>

<gadget id="calculator">1 - (1/11)</gadget>
<output>10/11 = around 0.909091</output>

<result>10/11 = around 0.909091</result>