(1/(13*15))+(1/(15*17))+(1/(17*19))+...+(1/(37*39)).
(1/(13*15))+(1/(15*17))+(1/(17*19))+…+(1/(37*39))．
<gadget id="calculator">1 / 2</gadget>
<output>1/2 = around 0.5</output>

<gadget id="calculator">1 / 13</gadget>
<output>1/13 = around 0.076923</output>

<gadget id="calculator">1 / 39</gadget>
<output>1/39 = around 0.025641</output>

<gadget id="calculator">(1/13) - (1/39)</gadget>
<output>2/39 = around 0.051282</output>

<gadget id="calculator">(1/2) * (2/39)</gadget>
<output>1/39 = around 0.025641</output>

<result>1/39 = around 0.025641</result>