There is a trapezoid whose upper base is 5 cm and lower base is 7 cm. If only the upper base is increased by 3 cm, then its area will increase by 4.5 cm2. Find the area of the original trapezoid?
有一个梯形，它的上底是5厘米，下底是7厘米，如果只把上底增加3厘米，那么它的面积就增加4.5平方厘米，求原来梯形的面积？
<gadget id="calculator">5 + 7</gadget>
<output>12</output>

<gadget id="calculator">2 / 3</gadget>
<output>2/3 = around 0.666667</output>

<gadget id="calculator">4.5 * (2/3)</gadget>
<output>3</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>