The upper and lower bases of the trapezoid are 4 cm and 8 cm respectively, and the height is 3 cm. What is the area of the trapezoid in square centimeters?
梯形的上底下底分别是4厘米、8厘米，高是3厘米，梯形的面积是多少平方厘米．
<gadget id="calculator">4 + 8</gadget>
<output>12</output>

<gadget id="calculator">3 / 2</gadget>
<output>3/2 = around 1.5</output>

<gadget id="calculator">12 * (3/2)</gadget>
<output>18</output>

<result>18</result>