If the upper base of a trapezoid is 5 cm, the lower base is 8 cm, and the distance between the upper and lower bases is 4 cm, what is the area of the trapezoid in square centimetres?
一个梯形的上底是5cm，下底是8cm，上下底之间的距离是4cm，那么这个梯形的面积是多少平方厘米．
<gadget id="calculator">5 + 8</gadget>
<output>13</output>

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

<gadget id="calculator">13 * 2</gadget>
<output>26</output>

<result>26</result>