The cross-section of a part is trapezoidal. The sum of the upper and lower bases of the trapezoid is equal to the height, which is 40 mm. What is the area of the cross-sectional area of this part in square millimeters?
一个零件的截面是梯形，梯形上、下底的和与高相等，都是40毫米，这个零件截面的面积是多少平方毫米．
<gadget id="calculator">40 / 2</gadget>
<output>20</output>

<gadget id="calculator">40 * 20</gadget>
<output>800</output>

<result>800</result>