(1/(1*3))+(1/(3*5))+(1/(5*7))+...+(1/(19*21)).
(1/(1*3))+(1/(3*5))+(1/(5*7))+…+(1/(19*21))．
<gadget id="calculator">1 / 21</gadget>
<output>1/21 = around 0.047619</output>

<gadget id="calculator">1 - (1/21)</gadget>
<output>20/21 = around 0.952381</output>

<gadget id="calculator">1 / 2</gadget>
<output>1/2 = around 0.5</output>

<gadget id="calculator">(20/21) * (1/2)</gadget>
<output>10/21 = around 0.47619</output>

<result>10/21 = around 0.47619</result>