trapezoid, the sum of the upper and lower bases is 8.5 cm, and the height is 4 cm, what is its area?
一个梯形，上底与下底的和是8.5厘米，高是4厘米，它的面积是多少．
<gadget id="calculator">4 / 2</gadget>
<output>2</output>

<gadget id="calculator">8.5 * 2</gadget>
<output>17</output>

<result>17</result>