A trapezoid, its upper base is 5cm long, its lower base is 7cm long, and its height is 4cm. Draw a triangle with the largest area inside the trapezoid. What is the area of this triangle in cm**2.
一个梯形，它的上底长5cm，下底长7cm，它的高是4cm，在这个梯形内部画一个面积最大的三角形，这个三角形的面积是多少cm**2．
<gadget id="calculator">1 / 2</gadget>
<output>1/2 = around 0.5</output>

<gadget id="calculator">(1/2) * 7 * 4</gadget>
<output>14</output>

<result>14</result>