A line is a set of all possible points lying on it. An equation of a line shows the relationship between the various points of the lines. Does this mean that any point can lie on the line? An equation of line sets the conditions for the points to satisfy. The equation of line is algebraic in nature consisting of variables x and y. By the equation of line, we can do some future estimation and can get to know about the types of lines.
Vertical Line
The equation of a vertical line at a distance v from the yaxis is either x = v or x = −v. Sign depends upon the position of the line.
Horizontal Line
The equation of a horizontal line at a distance h from the xaxis is either y = h or y = −h. Sign depends upon the position of the line.
PointSlope Form
This form of the equation gives the condition for a point to lie on the line. Suppose A (x0, y0) be a fixed point on the line l of slope m. Consider any other point, say B (x, y). The point B (x, y) will lie on the line l if and only if, its coordinates satisfy the equation
m = (y − y_{0}) / (x − x_{0}) (from the definition of slope)
i.e., y − y_{0} = m (x − x_{0})
What if two points are given? Let’s find out.
Two–Points Form
Imagine a line l passing through two points A (x1, y1) and B (x2, y2). Consider a point, say C (x, y). This point will satisfy the condition to lie on l if they satisfy the condition of collinearity.
Slope of AC = Slope of BC = Slope of AC
i.e., (y − y_{1}) / (x − x_{1}) = (y_{2} − y_{1}) / (x_{2} − x_{1}) or, (y − y_{1}) = (x − x_{1}) (y_{2} − y_{1}) / (x_{2} − x_{1}) is the required equation of the line.
SlopeIntercept Form
What is an intercept? A yintercept is a distance at which the line intersects the yaxis. An xintercept is a distance at which the line cuts the xaxis. Suppose a line l with slope m intersects the yaxis at a distance ‘c’ from the origin. The equation of line l passing through the point (0, c) is given by:
m = (y − c) / (x − 0) or, y = mx + c
Consider a line l with slope m intersects the xaxis at a distance ‘d’ from the origin. The equation of line l passing through the point (d, 0) is given by:
m = (y − 0) / (x − d) or, y = m(x − d)
If the line l intersects both the axes what will be the equation of the line?
Intercept Form
Suppose a line l with slope m intersect the yaxis at a distance ‘c’ from the origin and xaxis at a distance of ‘d’ from the origin. The equation of line l passing through point (0, c) and (d, 0) is:
(y − y_{1}) / (x − x_{1}) = (y_{2} − y_{1}) / (x_{2} − x_{1}) or, (y − 0) / (x − d) = (c − 0) / (0 − d)
Solving we get, y/c + x/d = 1.
Normal Form
Suppose we have a line l in XYplane. The perpendicular distance (normal) from origin to the line is known and the angle with the positive direction of the xaxis.
Here, length of normal OA = p and it makes an angle α with the xaxis. The coordinates of A will be (p cosα, p sinα). OA is perpendicular to l. Slope of line l = −1 / slope of OA = −1 / tan α. Solving we have the required equation of the line, x cos α + y sin α = p. Till now, we have seen that the equations of the line are linear equations in x and y and can be generalized.
General Equation of Line
The general equation for any line is Ax + By + C = 0, where x and y are variables; A and B are not zero simultaneously. It is interesting to know that the general equation of a line can be reduced into any form of the equations.
Slope–Intercept Form
If B ≠ 0, then Ax + By + C = 0 can be reduced to y = − (A/B) x − C/B (of the form y = mx + c). Here, the slope of the line, m = − A/B, and yintercept, c = −C/B. If B = 0, x = −C/A is the xintercept.
Intercept Form
If C ≠ 0, then Ax + By + C = 0 can be reduced to (x/ (− C/A)) + (y/ (− C/B)) = 1 (of the form x/a + y/b = 1). Here, xintercept = −C/A, and yintercept = −C/B. If C = 0, Ax + By = 0 ⇒ A = B = 0. It shows a line passing through origin.
Normal Form
The normal form of equation is x cosα + y sinα = p. Reducing it in the form of Ax + By + C = 0, we have cosα = ±A/(A^{2} + B^{2})^{1/2}; sin α = ±B/(A^{2} + B^{2})^{1/2} and p = ±C /(A^{2}+ B^{2})^{1/2}.
Solved Example
Problem: Find the slope and the x and yintercepts from the equation 4x + 5y − 8 = 0.
Solution: Given equation 4x + 5y − 8 = 0 can be rewritten as y = − (4/5) x + 8/5.
Comparing with y = mx + c, we have slope,
m = − 4/5, y–intercept, c = 8/5.
Comparing with y = m(x − d), we have
y = − 4/5(x − 2). So, xintercept = 2.
209 videos443 docs143 tests

1. What are the different forms of the equation of a line? 
2. How can I convert a line equation from slopeintercept form to pointslope form? 
3. What is the general form of a line equation and how is it derived? 
4. How can I convert a line equation from pointslope form to slopeintercept form? 
5. Can I convert a line equation directly from general form to slopeintercept form? 
209 videos443 docs143 tests


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