Calculation: (5/6)-(7/12)+(9/20)-(11/30)+…+((18+19)/(18*19))-((19+20)/( 19*20)).
计算：(5/6)-(7/12)+(9/20)-(11/30)+…+((18+19)/(18*19))-((19+20)/(19*20))．
<gadget id="calculator">1 / 2</gadget>
<output>1/2 = around 0.5</output>

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

<gadget id="calculator">(1/2) - (1/20)</gadget>
<output>9/20 = around 0.45</output>

<result>9/20 = around 0.45</result>