It is known that the product of the upper base and the height of a trapezoid is 12, and the product of the lower base and the height is 18. What is the area of the trapezoid?
已知一个梯形的上底与高的积是12，下底与高的积是18，这个梯形的面积是多少．
<gadget id="calculator">12 + 18</gadget>
<output>30</output>

<gadget id="calculator">30 / 2</gadget>
<output>15</output>

<result>15</result>