There are three line segments whose lengths are 5, 7, and 9 respectively. If they are used as the upper base, lower base, and height of a right-angled trapezoid, what is the maximum area of the trapezoid?
有三条分别长5、7、9的线段，用它们作为某个直角梯形的上底、下底和高，那么梯形的面积最大是多少．
<gadget id="calculator">5 + 7</gadget>
<output>12</output>

<gadget id="calculator">9 / 2</gadget>
<output>9/2 = around 4.5</output>

<gadget id="calculator">12 * (9/2)</gadget>
<output>54</output>

<result>54</result>