Question: I have two points store in two variable, which forms a line. I want to find a point which is perpendicular to that line from one end point in that line.

Suppose I have two points P1(x1,y1) and P2(x2,y2) then i want to find a third point P3 such that line(P1-P2) is perpendicular to line(P2,P3) and intersect at P2.

Answer: 
First, the angle:

```
public static double angle (double x1, double y1, double x2, double y2) {
    double xdiff = x1 - x2;
    double ydiff = y1 - y2;
    //double tan = xdiff / ydiff;
    double atan = Math.atan2(ydiff, xdiff);
    return atan;
}

```

To get the perpendicular, you must add PI/2 to the angle of the line defined by your two points.

Once you have that angle, the formula is:

```
x = interceptPt.x + sin(perp_angle) * distance;
y = interceptPt.y + cos(perp_angle) * distance;

```
You can store your points in `vec2d`, then use some mathematical equations to get the perpendicular point.

```
vec2d getPerpendicularPoint(vec2d A, vec2d B, float distance)
{
   vec2d M = (A + B) / 2;
   vec2d p = A - B;
   vec2d n = (-p.y, p.x);
   int norm_length = sqrt((n.x * n.x) + (n.y * n.y));
   n.x /= norm_length;
   n.y /= norm_length;
   return (M + (distance * n));
}

```