(4/(1*3))+(4/(3*5))+(4/(5*7))+...+(4/(19*21))=how much.
(4/(1*3))+(4/(3*5))+(4/(5*7))+…+(4/(19*21))=多少．
<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">2 * (20/21)</gadget>
<output>40/21 = around 1.904762</output>

<result>40/21 = around 1.904762</result>