A right-angled trapezoid, the sum of the upper and lower bases is 13cm, the two waists are 6cm and 9cm respectively, what is the area of the trapezoid in cm**2.
一个直角梯形，上底与下底之和是13cm，两腰分别是6cm和9cm，该梯形的面积是多少cm**2．
<gadget id="calculator">6 / 2</gadget>
<output>3</output>

<gadget id="calculator">13 * 3</gadget>
<output>39</output>

<result>39</result>