If a line makes an angle θ with x - axis in the positive direction, then θ is called inclination of the line and 'tan θ' is called slope of the line and is denoted by m.
* Slope of x - axis or any line parallel to it is "Zero".
* Slope of y - axis or any line parallel to it is "tan 90o" not defined.
* Slope of a line passing through two points
(x1, y1) and (x2, y2) is x1 ≠ x2
* Two lines are parallel if their slopes are equal.
* Two lines are perpendicular if product of their slopes is equal to '-1'.
* Equation of x - axis is y = 0
Equation of y - axis is x = 0
Equation of line parallel to x - axis is y = K (K = constant).
Equation of line parallel to y - axis is x = K (K = constant).
Equation of line passing through (x1, y1) and having slope 'm' is y - y1 = m(x - x1)
* Equation of line passing through (x1, y1) and (x2, y2) is
(y - y1) (x2- x1) = (y2 - y1) (x - x1).
* Equation of line having slope 'm' and Y - intercept 'c' is y = mx + c.
* Equation of line making X - intercept 'a' and y-intercept 'b' is
* ax + by + c = 0 is the general form of equation of a straight line with slope and intercepts , respectively on coordinate axes.
* Equation of line with inclination θ and passing through (x1, y1) is
(symmetric form) where θ (0, Π/2) U (Π/2, Π)
* x = x1 + r cos θ, y = y1 + r sin θ are the parametric equations of line passing through (x1, y1).