Question: How is possible to detect if a 3D point is inside a cone or not? 

```
Ross cone = (x1, y1, h1)
Cone angle = alpha
Height of the cone = H
Cone radius = R
Coordinates of the point of the cone = P1 (x2, y2, h2)
Coordinates outside the cone = P2( x3, y3, h3)

Result for point1 = true
Result for point2 = false

```

Answer: 
A cone is simply an infinite number of circles whose size is defined by a linear equation that takes the distance from the point. Simply check if it's inside the circle of the appropriate size.
Wouldn't it be easier to compute angle between vector to center of cone and vector from apex pointing at point under evaluation. If vector projection is used and the length of the resultant vector is shorter then the vector pointing at the center of the cone the between the angle and length you know if you are inside a cone.

<https://en.wikipedia.org/wiki/Vector_projection>