The upper base, lower base, and height of a trapezoid are integers in centimeters. The products of the upper base, lower base, and height are 12 and 21 respectively. What is the area of the trapezoid in square centimeters?
一个梯形的上底、下底、高都是整厘米数，上底、下底与高的乘积分别为12和21，这个梯形的面积是多少平方厘米．
<gadget id="calculator">12 + 21</gadget>
<output>33</output>

<gadget id="calculator">33 / 2</gadget>
<output>33/2 = around 16.5</output>

<result>33/2 = around 16.5</result>