The height of a trapezoid is 60 cm, the length of the lower base is twice as long as the upper base, and the length of the lower base is 12 cm. Find the area of the trapezoid. (First write out the letter formula, and then substitute the value into the formula to calculate)
一个梯形的高是60厘米，下底的长度是上底的2倍，下底长12厘米．求梯形的面积．(先写出字母公式，再把数值代入公式计算)
<gadget id="calculator">12 / 2</gadget>
<output>6</output>

<gadget id="calculator">6 + 12</gadget>
<output>18</output>

<gadget id="calculator">60 / 2</gadget>
<output>30</output>

<gadget id="calculator">18 * 30</gadget>
<output>540</output>

<result>540</result>