1-(1/(1+2))-(1/(1+2+3))-…-(1/(1+2+3+…+20)).
1-(1/(1+2))-(1/(1+2+3))-…-(1/(1+2+3+…+20))．
<gadget id="calculator">1 / 2</gadget>
<output>1/2 = around 0.5</output>

<gadget id="calculator">1 / 21</gadget>
<output>1/21 = around 0.047619</output>

<gadget id="calculator">(1/2) - (1/21)</gadget>
<output>19/42 = around 0.452381</output>

<gadget id="calculator">2 * (19/42)</gadget>
<output>19/21 = around 0.904762</output>

<gadget id="calculator">1 - (19/21)</gadget>
<output>2/21 = around 0.095238</output>

<result>2/21 = around 0.095238</result>