If the sum of the upper and lower bases of a trapezoid is a centimeter and the height is 4 centimeters, what is the area in square centimetres?
一个梯形上下底的和是a厘米，高是4厘米，则面积是多少平方厘米．
<gadget id="calculator">4 / 2</gadget>
<output>2</output>

<result>2</result>