Use two right-angled trapezoids with an upper base of 18cm, a lower base of 12cm, and a height of 9cm to form a large trapezoid. What is the area of the trapezoid?
用两个上底18cm，下底12cm，高9cm的直角梯形拼成一个大的梯形，这个梯形的面积是多少．
<gadget id="calculator">18 + 12</gadget>
<output>30</output>

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

<gadget id="calculator">30 * (9/2) * 2</gadget>
<output>270</output>

<result>270</result>