Equation of a line is the equation satisfied by all the points which lie on the line.
So, we can call every point on the line as a solution of the equation.
As there are infinitely many points on a line, the equation of a line has infinitely many solutions.
For every value of x, you can find a value for y by substituting the x value in the equation of the line.
That is, consider the line whose equation is 2x + y = 4.
We can find the value of y, when x equals 2.
Substituting x = 2 in 2x + y = 3, we have
2(2) + y = 3
? 4 + y = 3
? y = -1
So, (2, -1) is a point on the line 2x + y = 4.
There are various approaches to derive equation of a line.
Point Slope Form of Equation of a Line
We can draw a line if we know its slope and a point through which it passes. And the line we get is unique. Is it? Try.
Hence we can find the equation of a line when its slope m and a point P(x1, y1) through which passes are given.
The equation of the line is given as (y – y1) = m(x – x1)
Slope Intercept Form of Equation of a Line :
Suppose if c is the y-intercept of the line, then (0, c) is a point on the line.
So, the equation (y – y1) = m(x – x1) can be written as (y – c) = m(x – 0)
That is, y = mx + c
So, we can find the equation of the line if its slope and y-intercept are known.
Equation of a Line Passing through Two Given Points:
Equation of a line passing through two given points is also unique.
So, we can find the equation of a line, if we know any two points on it.
Suppose line l passes through the points A(x1, y1) and B(x2, y2), then slope of the line l is
m = y2 - y1 / x2 -x1
Now we know the slope and a point on the line (considering either A or B) and hence we can find the equation of the line using the formula (y – y1) = m(x – x1).
So, the equation of the line is
(y - y1) = y2 - y1 / x2 -x1 (x - x1)
OR
(y - y2) = y2 - y1 / x2 -x1 (x - x2)
That is y - y1/ x - x1 = y2 - y1 / x2 -x1
In general, we can rewrite the equation of a line in the form ax + by + c = 0, where a, b and c are real numbers.