Calculation: (1/(4*5))+(1/(5*6))+(1/(6*7))+...(1/(39*40)).
计算：(1/(4*5))+(1/(5*6))+(1/(6*7))+…(1/(39*40))．
<gadget id="calculator">1 / 4</gadget>
<output>1/4 = around 0.25</output>

<gadget id="calculator">1 / 40</gadget>
<output>1/40 = around 0.025</output>

<gadget id="calculator">(1/4) - (1/40)</gadget>
<output>9/40 = around 0.225</output>

<result>9/40 = around 0.225</result>