定义：弧长等于圆半径长的弧所对的圆心角为1弧度.1弧度约为57°17'44.806'',1°为π/180弧度,近似值为0.01745弧度,周角为2π弧度,平角（即180°角）为π弧度,直角为π/2弧度.