The sum of the upper and lower bases of a trapezoid is 28 cm, and the height is half of the sum of the upper and lower bases. What is the area of the trapezoid in square centimeters?
一个梯形，上底下底的和是28厘米，高是上下底和的一半，这个梯形的面积是多少平方厘米．
<gadget id="calculator">28 / 2</gadget>
<output>14</output>

<gadget id="calculator">14 / 2</gadget>
<output>7</output>

<gadget id="calculator">28 * 7</gadget>
<output>196</output>

<result>196</result>