Number of Solutions in a System of Equations

A linear equation in two variables is an equation of the form ax + by + c = 0 where a, b, c ∈ R, a, and b ≠ 0. When we consider a system of linear equations, we can find the number of solutions by comparing the coefficients of the equations. Also, we can find the number of solutions by graphical method. In this article, we will learn how to determine the number of solutions in a system of equations with two variables.

Three Types of Solutions of a System of Linear Equations

Consider the pair of linear equations in two variables x and y.

a1x + b1y + c1 = 0

a2x + b2y + c2 = 0

Here a1, b1, c1, a2, b2, c2 are all real numbers.

Note that, a12 + b12 ≠ 0, a22 + b22 ≠ 0

1. If (a1/a2) ≠ (b1/b2), then there will be a unique solution. If we plot the graph, the lines will intersect. This type of equation is called a consistent pair of linear equations. The graph is shown below.

2. If (a1/a2) = (b1/b2) = (c1/c2), then there will be infinitely many solutions. The lines will coincide. This type of equation is called a dependent pair of linear equations in two variables

3. If (a1/a2) = (b1/b2) ≠ (c1/c2), then there will be no solution. If we plot the graph, the lines will be parallel. This type of equation is called an inconsistent pair of linear equations.

In Short:

Condition Number of Solutions
(a1/a2) ≠ (b1/b2) Unique Solution
(a1/a2) = (b1/b2) = (c1/c2) Infinitely Many Solutions
(a1/a2) = (b1/b2) ≠ (c1/c2) No Solution

Example

How many solutions does the following system have?

y = -2x – 4

y = 3x + 3

Solution:

Given y = -2x – 4

y = 3x + 3

Rewriting to the general form

-2x – y – 4 = 0

3x – y + 3 = 0

Comparing the coefficients,

(a1/a2) = -⅔

(b1/b2) = -1/-1 = 1

(a1/a2) ≠ (b1/b2)

Hence, this system of equations will have only one solution.