Family of lines

Family of Lines


  • In coordinate geometry, a "family of lines" refers to a set of lines that share a common characteristic or relationship.
  • Lines in a family can vary in certain parameters while maintaining a particular connection or property.

General Form of Family of Lines:

  • A family of lines can be represented in a general form as:
    Here, λ represents a parameter that determines different lines within the family.
  • A family of lines is represented by an equation involving parameters:
    Here, λ represents the parameter that generates different equations within the family.
  • A family of lines is represented by an equation that involves a parameter 'm' (or other variables) indicating variations among the lines.
  • Example: y=mx+c, where 'm' represents the slope and 'c' represents the y-intercept.

Characteristics of a Family of Lines:

  1. Parameter Variation:

    • The parameter 'm' allows for variations among lines in the family.
    • Changing 'm' generates different lines within the family, each having a unique slope.
  2. Common Property:

    • Despite varying parameters, all lines in the family share a common property or relationship specified by the equation.

Examples of Families of Lines:

  1. Lines Passing Through a Point:

    • y=mx+2 represents a family of lines passing through the point (0, 2) with different slopes 'm'.
  2. Parallel Lines:

    • y=mx+4 and y=mx3 represent families of parallel lines with different y-intercepts but the same slope 'm'.
  3. Perpendicular Lines:

    • y=mx+5 and y=1mx+5 represent families of perpendicular lines sharing the same y-intercept '5' but with slopes 'm' and 1m.

Types of Families of Lines:

  1. General Equation of a Family:

    • Represents a set of lines characterized by a general equation involving one or more parameters.
    • Example: Ax+By+λ=0 represents a family of lines with the parameter λ.
  2. Variable Slope Family:

    • Lines in this family share a common intercept but can have varying slopes.
    • Example: y=mx+c where m represents the slope and c is the intercept; lines with different slopes but a common intercept belong to this family.
  3. Fixed Slope Family:

    • Lines in this family share a common slope but can have varying intercepts.
    • Example: y=2x+c represents lines with a fixed slope of 2 but different y-intercepts.
  4. Orthogonal Family:

    • Lines in this family are perpendicular to a particular line.
    • Example: If a line is represented by y=mx+c, lines perpendicular to it would belong to the orthogonal family, having slopes equal to 1/m.

Example of a Family of Lines:

Consider a family of lines represented by the equation 2x+3y+λ=0, where λ is the parameter.

  • For λ=1, the equation becomes 2x+3y+1=0.
  • For λ=2, the equation becomes 2x+3y2=0.
  • Each value of λ generates a different line within the family, while retaining the form 2x+3y+λ=0.


  • Families of lines offer a systematic way to describe multiple lines sharing a common property or relationship.
  • They provide a structured approach for understanding and analyzing various line configurations.


  • A family of lines represents a group of lines described by an equation containing a parameter that differentiates the individual lines within the family.
  • Understanding families of lines aids in recognizing patterns, relationships, and properties shared among multiple lines.