Calculation: (1990/(1990*1991))+(1990/(1991*1992))+...+(1990/(1999*2000)).
计算：(1990/(1990*1991))+(1990/(1991*1992))+…+(1990/(1999*2000))．
<gadget id="calculator">1 / 1_990</gadget>
<output>1/1_990 = around 0.000503</output>

<gadget id="calculator">1 / 2_000</gadget>
<output>1/2_000 = around 0.0005</output>

<gadget id="calculator">(1/1_990) - (1/2_000)</gadget>
<output>1/398_000 = around 0.000003</output>

<gadget id="calculator">1_990 * (1/398_000)</gadget>
<output>1/200 = around 0.005</output>

<result>1/200 = around 0.005</result>