The perimeter of a right-angled trapezoid is 50cm, and the two waists are 4cm and 5cm respectively. What is its area in cm**2.
直角梯形的周长是50cm，两条腰分别是4cm和5cm，它的面积是多少cm**2．
<gadget id="calculator">50 - 4 - 5</gadget>
<output>41</output>

<gadget id="calculator">4 / 2</gadget>
<output>2</output>

<gadget id="calculator">41 * 2</gadget>
<output>82</output>

<result>82</result>