The sum of the upper and lower bases of a trapezoid is 18 decimeters, which is twice the height. What is its area?
一个梯形的上、下底之和是18分米，是高的2倍，它的面积是多少．
<gadget id="calculator">18 / 2</gadget>
<output>9</output>

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

<gadget id="calculator">18 * (9/2)</gadget>
<output>81</output>

<result>81</result>