trapezoidal billboard, the upper base is 3 meters, the lower base is 5 meters, and the height is 2.5 meters. What is the area of this billboard in square meters?
一块梯形广告牌，上底是3米，下底5米，高2.5米．这块广告牌的面积是多少平方米？
<gadget id="calculator">3 + 5</gadget>
<output>8</output>

<gadget id="calculator">2.5 / 2</gadget>
<output>1.25</output>

<gadget id="calculator">8 * 1.25</gadget>
<output>10</output>

<result>10</result>