Computational Geometry

1.//If endpoint of one diagonal of a square is known,find the endpoint of another diagonal 

if (x1,y1),(x3,y3) are known,finding (x2,y2) and (x4,y4)
https://www.quora.com/Given-two-diagonally-opposite-points-of-a-square-how-can-I-find-out-the-other-two-points-in-terms-of-the-coordinates-of-the-known-points
static void square()
    {
        x[2] = (x[1]+x[3]+y[1]-y[3])/2;
        x[4] = (x[1]+x[3]-y[1]+y[3])/2;
        y[2] = (x[3]-x[1]+y[1]+y[3])/2;
        y[4] = (x[1]-x[3]+y[1]+y[3])/2;
        
        area_of_square = Math.pow(x[1]-x[2],2) + Math.pow(y[1]-y[2],2);
        
    } 
 
 
 
 
2.//area of a triangle if three endpoints of triangle are known
A(x1,y1), B(x2,y2), C(x3,y3) are known (vertices of triangle)
    area_of_triangle = Math.abs( x1*(y2 - y3) + x2*(y3 - y1) + x3*(y1 - y2) )/2
    
    
    
    
3.//How to check if a point A lies inside triangle
P(a,b) is known 
A(x1,y1) , B(x2,y2)  , C(x3,y3) are known (vertices of triangle)

if(area_of_triangle(ABP) + area_of_triangle(ACP) + area_of_triangle(BCP) == area_of_triangle(ABC))
point P lies in triangle.