A radian is the Angle subtended by an arc of a circle whose length is just equal to the radius.
一弧度就是长度刚好等于半径的一段圆弧所对的圆心角。
2
The Angle subtended at the center of a sphere by an area on its surface numerically equal to the square of the radius.
在单位半径的球体中心对球表面上的投影面积所张的角。
3
We solved at the same time a pure mathematical problem by physical procedure i. e. calculating the solid angle subtended by a circular plate to any point in the space.