在 @王大锤 的提点下，去搜索了英文维基，果然有解释。 Etymology of the arc- prefix When measuring in radians, an angle of θ radians will correspond to an arc whose length is rθ, where r is the radius of the circle. Thus, in the unit circle, "the arc whose cosine is x" is the same as "the angle whose cosine is x", because the length of the arc of the circle in radii is ...
