For the line x+y−2=0 1,2,1,−2 , the points

Suppose that L=0 be a line. Suppose that A,B are any two points.  Substitute the points in the equation of the line L=0, the values obtained are LA,LB If the values LA,LB are of same sign, then the points A,B lies on the same side of the line L=0 If the values LA,LB are of opposite sign, then the points A,B lies on the opposite side of the line L=0