- A straight line is represented by an equation of the first degree in two variables (x and y). Conversely locus of an equation of the first degree in two variables is a straight line.
- A straight line is completely determined by its slope (direction) and a point is given through which the line must pass.
Equation of x-axis:
- If the y-coordinates of each point on the x-axis is zero (0) and if P (x, y) is any point on x-axis, then y=0.
Equation of y-axis:
- As the x-coordinates of each point on the
y-axis is zero (0) and if P (x, y) is a point on y-axis, then x = 0.
Equation of a line parallel to y-axis:
- Let LM be a straight line parallel to y-axis at a distance k from the y-axis. Then, the abscissa (the x-coordinate), x=k. Hence, the equation of the line parallel to y-axis is x = k.
Equation of a line parallel to x-axis:
- Let LM be the line parallel to y-axis at a distance l from the x-axis. Then, the y-coordinate (ordinate) of each point on LM is l. Then, y = l is the equation to a line parallel to x-axis.
Remark:
i) If a line is parallel to y-axis is at a distance of k units from the x-axis, then the equation to the line is x = -k.
ii) If a line parallel to x-axis at a distance of l units from the y-axis, then the equation to the line is y = - l.