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;

```
If you want to use Java I can recommend to use [JTS](https://github.com/locationtech/jts). Create a [LineSegment](https://locationtech.github.io/jts/javadoc/org/locationtech/jts/geom/LineSegment.html) and use the pointAlongOffset method. Given Points p1 and p2 the code would look like that:

```
// create LineSegment
LineSegment ls = new LineSegment(p1.getX(), p1.getY(), p2.getX(), p2.getY());
// perpendicular distance to line
double offsetDistance = 10;
// calculate Point right to start point
Coordinate startRight = ls.pointAlongOffset(0, offsetDistance);
// calculate Point left to start point
Coordinate startLeft = ls.pointAlongOffset(0, -offsetDistance);
// calculate Point right to end point
Coordinate endRight = ls.pointAlongOffset(1, offsetDistance);
// calculate Point left to end point
Coordinate endLeft = ls.pointAlongOffset(1, -offsetDistance);

```