Example of what I'm taking about

My question will be clearer looking at the picture. The dash area is the area I want to know if the vector is inside of it or not. With the picture drawn out, the question seems to be trivial but I want to have a mathematical answer. My end goal is to have this problem solve through programming.


-The circle O(xc, yc) has a radius of R.

-The vector u(a,b) with the starting point of (xu, yu)

  • $\begingroup$ You need to specify what is given. Also, there are a lot of vectors that are equal to the one in the picture but have a different starting point. Do you care about that? $\endgroup$ – Vasya May 23 '19 at 3:51
  • $\begingroup$ @Vasya Thanks for the comment. I'm trying to have a general answer for an arbitrary vector and arbitrary circle in XY plan. I have added some more details in the question. $\endgroup$ – Anh Tran May 23 '19 at 4:10
  • $\begingroup$ Is it $\mathbf u$ that you wish to test? Is $(x_u,y_u)$ the cone’s apes? $\endgroup$ – amd May 23 '19 at 5:40
  • $\begingroup$ @amd Yes, u is the vector I want to test and $(x_u, y_u)$ is the pointy end. $\endgroup$ – Anh Tran May 23 '19 at 7:05

enter image description here

Because triangles OAP and OBP are right,

$$x^2+R^2=OP^2, \mbox{or} $$ $$x=\sqrt{OP^2-R^2}$$

From properties of right triangles:

$$\frac{x}{OP}=\cos\frac{\alpha}{2},$$ where $\alpha$ denotes the angle BPA. Then


Because triangles BPO and APO have a side equal to R and share the side OP, then BPO and APO are identical and therefore OP cuts $\alpha$ into two equal angles $\frac{\alpha}{2}$.

Now plot an arbitrary vector $\vec{PC}$ (the coordinates of C are assumed known). The vectors $$\frac{\vec{PC}}{PC} \mbox{ and } \frac{\vec{PO}}{PO}$$ are unit vectors.


$$\frac{\vec{PC}}{PC} \cdot \frac{\vec{PO}}{PO}=\cos\gamma.$$

If $\vec{PC}$ is between the two tangents,

$$\cos\gamma \ge \cos\frac{\alpha}{2},$$ because $\gamma \le \frac{\alpha}{2}$.

Therefore the condition for an arbitrary vector $\vec{PC}$ drawn from P and ending in C, to be between the two tangents, is

$$\frac{\vec{PC}}{PC} \cdot \frac{\vec{PO}}{PO} \ge \frac{\sqrt{OP^2-R^2}}{OP^2}$$.

To ensure the vector does not cross the circle, add these two conditions:

$$OC \ge R$$

and $PC \le PA$, that is

$$PC \le \sqrt{OP^2-R^2}$$

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  • $\begingroup$ Good job, beautiful equation, easy to program. I haven't check out the math yet but looks good so far. $\endgroup$ – Anh Tran May 24 '19 at 1:43

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