# Formulas and Results of Straight Lines

Consider two points and , then:

**01.** The distance formula

**02.** The midpoint formula

**03.** The point dividing in the ratio is

**04.** The slope of is

**05.** The slope of the x-axis = zero

**06.** The slope of a line parallel to the x-axis = zero

**07.** The slope of the y-axis is not defined, i.e.

**08.** The slope of a line parallel to the y-axis is not defined, i.e.

**09.** The equation of the x-axis is

**10.** The equation of the x-axis is

**11.** The equation of the line parallel to the x-axis and at a distance is.

**12.** The equation of the line parallel to the y-axis and at a distance is.

**13.** The equation of the line with slope and y-intercept is , which is called the slope – intercept form.

**14.** The equation of the line passing through and having the slope is , which is called the slope – point form.

**15.** The equation of the line passing through two points and is

.

**16.** The equation of the line having and as the x – intercept and y – intercept is and is called the equation of the line in intercept form.

**17.** The normal form of the straight line is , where is the length of the perpendicular from to the line, and is the inclination of the perpendicular.

**18.** The general form of the equation of a straight line is . Consider two lines and having the slopes and , respectively.

**19.** If two lines and are parallel, then .

**20.** If two lines and are parallel, then .

**21.** The angle from to is

**22.** The distance of point from the line is

.

**23.** If with is the equation of the line , the lies:

**(1)** Above the line if

**(2)** Below the line if

**(3)** On the line if

**24.** Three lines , , are concurrent if