The cross-section of a barrage is trapezoidal, with a width of 4 cm at the top, a width of 7 cm at the bottom, and a height of 1.8 cm. Find the cross-sectional area of the barrage.
一条拦河堤的横截面是梯形，上面宽4cm，下面宽7cm，高1.8cm，求这个拦河坝的横截面面积．
<gadget id="calculator">4 + 7</gadget>
<output>11</output>

<gadget id="calculator">1.8 / 2</gadget>
<output>0.9</output>

<gadget id="calculator">11 * 0.9</gadget>
<output>9.9</output>

<result>9.9</result>