The abscissa at the endpoint of an arc of a unit circle centered at the origin of a Cartesian coordinate system, the arc being of length x and measured counterclockwise from the point(1,0) if x is positive or clockwise if x is negative.

• 余弦以笛卡儿坐标系原点为圆心的单位圆的圆弦端点的横坐标，弧的长度为x，如果x为正，弧的长度则从（1，0）点按逆时针方向算起，若x为负，则按顺时针方向从（1，0）点算起