The radius of the circle is 2cm, then the arc length 1= how many cm is the arc subtended by the central angle of 180°.
圆半径为2cm，那么180°的圆心角所对的弧长1=多少cm．
<gadget id="calculator">3.14 * 2</gadget>
<output>6.28</output>

<result>6.28</result>