There is a trapezoidal piece of paper with an upper base of 4 cm, a lower base of 7 cm, and a height of 5 cm. What is its area in square centimeters?
有一块上底是4厘米，下底是7厘米，高是5厘米的梯形纸片，它的面积是多少平方厘米．
<gadget id="calculator">4 + 7</gadget>
<output>11</output>

<gadget id="calculator">5 / 2</gadget>
<output>5/2 = around 2.5</output>

<gadget id="calculator">11 * (5/2)</gadget>
<output>55/2 = around 27.5</output>

<result>55/2 = around 27.5</result>