# Substitution Method

How to solve a system of linear equations and inequalities by using their graphs: 3 examples and their solutions.

## Examplex - y = 4, 2x + y = 5

Choose one of the equation.

Let's choose x - y = 4.

Change it to [x = ...] form.

Then x = y + 4.

You can also change it to [y = ...] form.

Put x = y + 4

into the other equation 2x + y = 5.

Then 2(y + 4) + y = 5.

Solve the equation 2(y + 4) + y = 5.

Then y = -1.

Put this y = -1

into x = y + 4.

Then x = (-1) + 4.

-1 + 4 = 3

So x = 3.

y = -1

x = 3

So

x = 3, y = -1

is the answer.

## Examplex - y = 4, 2x - 2y = 8

Choose one of the equation.

Let's choose x - y = 4.

Change it to [x = ...] form.

Then x = y + 4.

Put x = y + 4

into the other equation 2x - 2y = 8.

Then 2(y + 4) - 2y = 8.

Solve the equation 2(y + 4) - 2y = 8.

Then the variable is removed

and you get 0 = 0.

0 = 0 is always true.

Just like this case,

when you get an equation

that is always true,

then the system has

infinitely many solutions.

So

infinitely many solutions

is the answer.

## Examplex - y = 4, x - y = -3

Choose one of the equation.

Let's choose x - y = 4.

Change it to [x = ...] form.

Then x = y + 4.

Put x = y + 4

into the other equation x - y = -3.

Then (y + 4) - y = -3.

Solve the equation (y + 4) - y = -3.

Then the variable is removed

and you get 4 = -3.

4 = -3 is always false.

Just like this case,

when you get an equation

that is always false,

then the system has

no solution.

So

no solution

is the answer.