The title does not say much. But my actual question is as follows. Let us suppose a line ax+by=0. This means A.B = 0 with A and B being vectors, where A = [a,b] and B = [x,y]. Therefore, A is perpendicular to B. Now my question is if A is also perpendicular to line ax+by=0 (I'm not sure if this makes sense). I read this statement in a book and I'm not able to understand how this came to be. Please help me understand this. Thanks in advance.