What is the largest area of a rectangle with a perimeter of 18?
周长是18的矩形的最大面积是多少．
<gadget id="calculator">18 / 4</gadget>
<output>9/2 = around 4.5</output>

<gadget id="calculator">(9/2) * (9/2)</gadget>
<output>81/4 = around 20.25</output>

<result>81/4 = around 20.25</result>