(1/(10*11))+(1/(11*12))+(1/(12*13))+...+(1/(19*20))=how much.
(1/(10*11))+(1/(11*12))+(1/(12*13))+…+(1/(19*20))=多少．
<gadget id="calculator">1 / 10</gadget>
<output>1/10 = around 0.1</output>

<gadget id="calculator">1 / 20</gadget>
<output>1/20 = around 0.05</output>

<gadget id="calculator">(1/10) - (1/20)</gadget>
<output>1/20 = around 0.05</output>

<result>1/20 = around 0.05</result>