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# Equation of  straight lines :

### 1.General form

The first-degree equation in x and y is known as a general form of a straight line.

Example –  ax + by + c = 0 , where  a and b are not both zero at a same time.

x = 0 , Y-axis.

y = 0 , X-axis

x = a , parallel to Y-axis.

Y = b, parallel to X-axis.

The distance of a point (x, y ) from x-axis is |y| and from y-axis is |x|.

Slope = -a/b

Intercept on x-axis :

Put  y=0.

x = -c/a

Intercept on y-axis:

Put x = 0

y = -c/b

### 2.Slope form

An equation of a line has slope m = tanθ and cutting off an intercept c on Y-axis.

Then

y = x.tanθ + c

y = mx+c

### 3. Intercept form

An equation of a straight line L  and cutting of an intercepts a and b upon axes of X and Y respectively.

Then

x/a + y/b = 1

if the equation of a straight line L is –

a) parallel to X-axis, X-intercept is not defined.

b) parallel to Y-axis, Y-intercept is not defined. ### 4.Point-slope form

An equation of a line passing through one point and having slope m.

Then

y-y1 = m.(x-x1).

Here m= tanθ ### 5.Two-point form

An equation of a straight line passing through two points (x1, y1) and (x2, y2).

Then

y – y1 = [ (y2 – y1)/( x2 -x1 )]( x – x1)  ### 6.Normal form

An equation of the straight line at which the perpendicular from the origin is of length p and makes an angle a with the positive direction of the x-axis is –

Then equation of a straight line is:  x cos a  +   y sin a  = p

OM = P ### 7.Parametric form—

An equation of a straight line passing through A( x1, y1 ) and making an angle θ with the positive direction of the x-axis. In this equation r is the directed distance between the points A( x1, y1 ) and P( x, y ).

x – x1 = r cosθ

x = x1 + r cosθ

Similarly ,

y – y1 = r sinθ

y = y1 + r sinθ

So, co-ordinate of any point on the line at a distance r from point A( x1, y1 )

can be taken as  ( x1 + r cosθ , y1 + r sinθ )

Hence r is positive for above points A and negative for points below A and 0< θ ≤ 2𝜋