The upper base of a trapezoid is 3 decimeters, the lower base is 5 decimeters, and the height is 4 decimeters. What is its area?
一个梯形的上底是3分米，下底是5分米，高是4分米，它的面积是多少平方分米．
<gadget id="calculator">3 + 5</gadget>
<output>8</output>

<gadget id="calculator">4 / 2</gadget>
<output>2</output>

<gadget id="calculator">8 * 2</gadget>
<output>16</output>

<result>16</result>