Home/ Questions/Q 8887961
In Process

The Archive Base Latest Questions

Editorial Team
  • 0
Asked: 2026-06-14T21:56:02+00:00

I have a triangle ABC inscribed in a circle. Point B is located in

  • 0

I have a triangle ABC inscribed in a circle. Point B is located in the centre of the circle, A and C are two points on the circle.

circle with inscribed triangle

Given

Given AB (length)

Given coords of A and B

Given angle B (angle ABC)

Needed

Find coords of C

What I know

AB = BC, both are radius’s

What I am using this for

I am making a basic 3D java game, for android. This will be used for looking left and right, so if you click on the right part of the screen the objects will move around you by adding one degree to angle B.

The code I tried for finding coords of C

rect.get(index)[5] = (int) ((di * Math.cos(Math.toRadians(angle-90)))+.5);
rect.get(index)[6] = (int) ((di * Math.sin(Math.toRadians(angle-90)))+.5);
rect.get(index)[5] = shapes x coord 
rect.get(index)[6] = shapes y coord 

di = radius length 
angle = angle B

and I added the .5 so that when the coord is truncated it is rounded.

My complete code

    double di = distance(playerx, playery, rect.get(index)[5], rect.get(index)[6]);
    double side1 = di;
    System.out.println("Side1: "+ side1);
    double side2 = side1;
    System.out.println("Side2: "+ side2);
    double side3 = distance(rect.get(index)[5], rect.get(index)[6], playerx, playery+di);
    System.out.println("Side3: "+ side3);

    double angle = ((side1*side1)+(side2*side2)-(side3*side3));
    angle = angle/(2*side1*side2);
    angle = Math.acos(angle)*(180/Math.PI);
    System.out.println("Angle: "+angle);
    if(playerx > rect.get(index)[5]){
        if(lookdirection.equals("left")){
            angle += 5;
        }
        if(lookdirection.equals("right")){
            angle -= 5;
        }
    }
    else{
        if(lookdirection.equals("left")){
            angle -= 5;
        }
        if(lookdirection.equals("right")){
            angle += 5;
        }
    }
    System.out.println("Angle: "+angle);

    rect.get(index)[5] = -(di * Math.cos(Math.toRadians(angle-90)));
    rect.get(index)[6] = -(di * Math.sin(Math.toRadians(angle-90)));

    di = distance(playerx, playery, rect.get(index)[5], rect.get(index)[6]);
    side1 = di;
    System.out.println("Side1: "+ side1);
    side2 = side1;
    System.out.println("Side2: "+ side2);
    side3 = distance(rect.get(index)[5], rect.get(index)[6], playerx, playery+di);
    System.out.println("Side3: "+ side3);
    angle = ((side1*side1)+(side2*side2)-(side3*side3));
    angle = angle/(2*side1*side2);
    angle = Math.acos(angle)*(180/Math.PI);
    System.out.println("Angle: "+angle);

    repaint();
}

The angles are now working but the X and Y coords
but

    rect.get(index)[5] = -(di * Math.cos(Math.toRadians(angle-90)));
rect.get(index)[6] = -(di * Math.sin(Math.toRadians(angle-90)));

are getting very large/ small values. They should stay ‘di’ distance away from point B.

Share

Leave an answer

You must login to add an answer.

Need An Account,

1 Answer

  1. Editorial Team

    UPDATED

    It seems that this is a geometry problem!
    I haven’t tested your code but I guess what you want is not

    rect.get(index)[6] = (int) ((di * Math.sin(Math.toRadians(angle-90)))+.5);

    but

    rect.get(index)[6] = (int) (-(di * Math.sin(Math.toRadians(angle-90)))+.5);

    Try to visualise what you are doing and check the identities under symmetry, shifts, and periodicity here. The image is turning and reflecting along the x axis with what you are doing now. You have to take care of rounding issues as well. If coordinates are close to 0 the image might not move.

    My suggestion is to keep your coordinates as doubles and only round them when you are going to render them into a pixel without overwriting their value

    • 0