0 a straight line that touches but does not cut into a curve -- 切线
1 (in a triangle that has one angle of 90°) the ratio of the length of the side opposite an angle less than 90° divided by the length of the shorter of the two sides that are next to the angle -- 正切
Similarly, the second of equations (30) yields equation (32) and the inequality (33)with the tangent replaced by the minus cotangent.
For a given trajectory (that is, given the initial conditions at the source), the maximum possible fundamental occurs when the trajectory has a vertical tangent.
The concept of visible tangent links can be extended to arbitrary shaped obstacles.
The tangent vectors are scaled to a fixed length.
Instead of using the action on tangent vectors, we use the action on pairs of tangent vectors.
In particular, it will become important to show that their unit tangent vectors form a set of measure zero in the unit tangent bundle.
We will also need to compare tangent spaces at nearby points.
In order to make it competitive it must be combined with some other algorithm that reduces the number of tangent distance comparisons to make.
