Hi I have two (two dimentional) linear equations (in the plane). and i am required to find out the angle between them. I have found the solution somewhere. the solution uses two vectors orthogal to the two lines. The problem now i have is: How to determine those orthogonal vectors? e.g: two equations are: 3x + y = 5 and 2x - y =4 Now the solution goes like this: n1 = 3i + j and n2 = 2i - j. Then the angle between them is found out by using Dot product. This is the required angle. How did they determine n1 and n2?