# Summary

- Slope of a line joining by .
- If a line makes an angle with the positive direction of ,
- Slope of is zero, slope of is not defined.
- Parallel lines have same slope.
- If two lines are perpendicular, the product of their slopes = (âˆ’ 1).
- The acute angle between lines with slopes is given by .
- Three points A, B, C are collinear if slope of AB = slope of BC.
- The equation of is and that of is
- Any line parallel to at a distance units from it is given by any line parallel to at a distance of units from it has equation
- The general equation of a line is of the form where are not zero simultaneously. Slope of this line is .
- If is the slope and is the -intercept of a line, then its equation is
- The equation of a line having slope and passing through is given by: .
- The equation of a line joining the points
- A line with -intercept units and -intercept units has equation:
- The equation of a line having normal distance from the origin and the angle between the normal and the positive is .
- The perpendicular distance of a line from a point is given by

The perpendicular distance of (0, 0) from - The distance between the parallel lines and is given by
- If two straight lines are parallel, then the coefficients of are proportional in their equations. In particular, the equations of two parallel lines differ only by a constant.
- The equation of straight line perpendicular to the line is of the form where is a constant.
- The equation of a straight line passing through the intersection of and is where is a constant.