Straight Lines in a cartesian plane is described algebraically by linear equations.

### Slope of a Line

Slope (m) of a non-vertical line passing through the points (x_{1}, y_{1}) and (x_{2}, y_{2}) is given by

m = (y_{2} - y_{1})/(x_{2} - x_{1})

If a line makes an angle α with the positive direction of x-axis, then the slope of the line is given by m = tan α. Slope of horizontal line is zero and slope of vertical line is undefined.

**Parallel and Perpendicular Lines**

Two lines are parallel if and only if their slopes are equal. Two lines are perpendicular if and only if product of their slopes is -1.

**Collinear Points**

Three points A, B and C are collinear, if and only if slope of AB = slope of BC.

### Equation of Line

Equation of the horizontal line having distance a from the x-axis is either y = a or y = - a.

Equation of the vertical line having distance b from the y-axis is either x = b or x = - b.

### Slope Intercept Form

The point (x, y) on the line with slope m and y-intercept c:

y = mx + c

### Two Point Form

Equation of the line passing through the points (x_{1}, y_{1}) and (x_{2}, y_{2}) is given by

y - y_{1} = {(y_{2} - y_{1})/{x_{2} - x_{1})}(x - x_{1})

### Point Slope Form

The point (x, y) lies on the line with slope m and through the fixed point (x_{1}, y_{1}):

y - y_{1} = m(x - x_{1})

### Intercept Form

Equation of a line making intercepts a and b on the x-and y-axis respectively:

x/a + y/b = 1