Use two identical trapezoids to form a parallelogram with a base length of 18dm and a height of 9dm, find the area of each trapezoid.
用两个完全相同的梯形拼成一个底边长为18dm，高为9dm的平行四边形，求每个梯形的面积．
<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>