For any non-zero natural number x, y, the prescribed symbol "△" represents an operation, and x△y=((2xy)/(x+y)), then 4△5=how much.
对于任意非零自然数x，y，规定符号"△"表示一种运算，且x△y=((2xy)/(x+y))，则4△5=多少．
<gadget id="calculator">2 * 4 * 5</gadget>
<output>40</output>

<gadget id="calculator">4 + 5</gadget>
<output>9</output>

<gadget id="calculator">40 / 9</gadget>
<output>40/9 = around 4.444444</output>

<result>40/9 = around 4.444444</result>