The cross section of a river is a trapezoid. The upper base of the trapezoid is 3 meters, the lower base is 10 meters, and the height is 2 meters. What is the area of the trapezoid cross section in square meters?
一条河的横截面是一个梯形，梯形的上底是3米，下底是10米，高是2米，求这个梯形横截面的面积是多少平方米？
<gadget id="calculator">3 + 10</gadget>
<output>13</output>

<gadget id="calculator">2 / 2</gadget>
<output>1</output>

<gadget id="calculator">13 * 1</gadget>
<output>13</output>

<result>13</result>