The sum of the upper and lower bases of a trapezoid is 42 cm and the height is 12 cm. What is the area in square centimeters?
一个梯形的上底与下底的和是42cm，高是12cm，面积是多少平方厘米？
<gadget id="calculator">12 / 2</gadget>
<output>6</output>

<gadget id="calculator">42 * 6</gadget>
<output>252</output>

<result>252</result>