The bottom of a national musical instrument used by an actor is trapezoidal. Its upper bottom is 80cm, its lower bottom is 120cm, and its height is 50cm. What is the area of the bottom of this musical instrument?
一位演员所用的民族乐器的底面是梯形，量得它的上底是80cm，下底是120cm，高为50cm，则此乐器的底面面积是多少？
<gadget id="calculator">80 + 120</gadget>
<output>200</output>

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

<gadget id="calculator">200 * 25</gadget>
<output>5_000</output>

<result>5_000</result>