The vector w = ai + bj is perpendicular to the line ax + by = c and parallel to the line bx â ay = c. It is also true that the acute angle between intersecting lines that do not cross at right angles is the same as the angle determined by vectors that are either normal to the lines or parallel to the lines. Use this information to find the acute angle between the lines below. yx + 9y = 0â, â4x + 5y = 3