(7/(2*3*4*5))+(9/(3*4*5*6))+…+(37/(17*18*19*20))+(39/(18* 19*20*21))=how much.
(7/(2*3*4*5))+(9/(3*4*5*6))+…+(37/(17*18*19*20))+(39/(18*19*20*21))=多少．
<gadget id="calculator">2 * 4</gadget>
<output>8</output>

<gadget id="calculator">1 / 8</gadget>
<output>1/8 = around 0.125</output>

<gadget id="calculator">19 * 21</gadget>
<output>399</output>

<gadget id="calculator">1 / 399</gadget>
<output>1/399 = around 0.002506</output>

<gadget id="calculator">(1/8) - (1/399)</gadget>
<output>391/3_192 = around 0.122494</output>

<result>391/3_192 = around 0.122494</result>