The upper base of a trapezoid is 7 cm, the lower base is 5 cm, and the height is 4 cm. What is its area in square centimeters?
一个梯形的上底是7厘米，下底是5厘米，高是4厘米，它的面积是多少平方厘米．
<gadget id="calculator">7 + 5</gadget>
<output>12</output>

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

<gadget id="calculator">12 * 2</gadget>
<output>24</output>

<result>24</result>