If the radius of a sector is 4 cm and the arc length subtended by the central angle is 9 cm, what is the area of the sector?
若一个扇形的半径是4厘米，圆心角所对的弧长是9厘米，则这个扇形的面积是多少．
<gadget id="calculator">405 * 16</gadget>
<output>6_480</output>

<gadget id="calculator">6_480 / 360</gadget>
<output>18</output>

<result>18</result>