There is a trapezoidal vegetable field, the upper base is 13 meters, 8 meters shorter than the lower base, and the height is 50 meters. What is the area of this trapezoidal vegetable field?
有一块菜地为梯形，上底是13米，比下底短8米，高是50米，这个梯形菜地的面积是多少？
<gadget id="calculator">13 + 8</gadget>
<output>21</output>

<gadget id="calculator">13 + 21</gadget>
<output>34</output>

<gadget id="calculator">50 / 2</gadget>
<output>25</output>

<gadget id="calculator">34 * 25</gadget>
<output>850</output>

<result>850</result>