Introduction to Straight Lines

Introduction to Straight Lines in Two-Dimensional Coordinate Geometry

Cartesian Coordinate System:

  • Two-dimensional space is represented using a Cartesian coordinate system, consisting of x-axis and y-axis intersecting at the origin (0,0).
  • Points on this plane are represented by ordered pairs (x, y), where 'x' denotes the horizontal distance and 'y' denotes the vertical distance from the origin.

Equation of a Straight Line (Linear Equation):

  • A straight line in a Cartesian plane can be defined by the equation y = mx + c or Ax + By + C = 0, where:
    • 'm' represents the slope of the line (rate of change of y with respect to x).
    • 'c' represents the y-intercept (where the line crosses the y-axis).
    • 'A', 'B', and 'C' are coefficients in the general form of the equation.

Slope-Intercept Form:

  • The slope-intercept form of the equation is y = mx + c, where 'm' is the slope and 'c' is the y-intercept.
  • The slope 'm' indicates the direction and steepness of the line:
    • Positive slope: Line slants upwards from left to right.
    • Negative slope: Line slants downwards from left to right.
    • Zero slope: Line is horizontal.
    • Undefined slope: Line is vertical.

Point-Slope Form:

  • The point-slope form of the equation is y - y₁ = m(x - x₁), where 'm' is the slope and (x₁, y₁) is a point on the line.
  • This form allows for easily deriving the equation when the slope and a point on the line are known.

Parallel and Perpendicular Lines:

  • Parallel lines have the same slope but different y-intercepts.
  • Perpendicular lines have slopes that are negative reciprocals of each other (the product of their slopes is -1).


  • x-intercept: The point where the line crosses the x-axis (y = 0).
  • y-intercept: The point where the line crosses the y-axis (x = 0).

Distance between Points:

  • The distance 'd' between two points (x₁, y₁) and (x₂, y₂) is given by the formula:

Angle between Lines:

  • The angle 'θ' between two lines with slopes 'm₁' and 'm₂' is given by the formula:


  • Straight lines find applications in various fields such as engineering, physics, computer graphics, and architecture.
  • They are used to model and analyze many real-world phenomena, including motion, electricity, and geometry of shapes.


  • Equations of straight lines are foundational in coordinate geometry, describing the relationship between points and their graphical representation on a Cartesian plane.
  • Different forms of equations allow for diverse methods to describe and analyze straight lines, facilitating problem-solving in various contexts.