(1/(1*5))+(1/(5*9))+(1/(9*13))+(1/(13*17))+(1/(17*21)).
(1/(1*5))+(1/(5*9))+(1/(9*13))+(1/(13*17))+(1/(17*21))．
<gadget id="calculator">1 * 5</gadget>
<output>5</output>

<gadget id="calculator">1 / 5</gadget>
<output>1/5 = around 0.2</output>

<gadget id="calculator">5 * 9</gadget>
<output>45</output>

<gadget id="calculator">1 / 45</gadget>
<output>1/45 = around 0.022222</output>

<gadget id="calculator">9 * 13</gadget>
<output>117</output>

<gadget id="calculator">1 / 117</gadget>
<output>1/117 = around 0.008547</output>

<gadget id="calculator">13 * 17</gadget>
<output>221</output>

<gadget id="calculator">1 / 221</gadget>
<output>1/221 = around 0.004525</output>

<gadget id="calculator">17 * 21</gadget>
<output>357</output>

<gadget id="calculator">1 / 357</gadget>
<output>1/357 = around 0.002801</output>

<gadget id="calculator">(1/5) + (1/45) + (1/117) + (1/221) + (1/357)</gadget>
<output>5/21 = around 0.238095</output>

<result>5/21 = around 0.238095</result>