# Substitution Method Algebra – Step-by-Step Examples and Solved Problems

The substitution method in algebra is a powerful tool for solving systems of equations. When I encounter two equations with two unknown variables, I can use this method to solve for both variables by isolating one variable in one equation and then substituting the result into the other equation.

For example, if I have the system of equations \begin{align*} y &= 2x + 3 \ x + y &= 7 \end{align*}, I can solve for $y$ in the first equation and then substitute that expression for $y$ in the second equation to find the value of $x$. This process transforms a system of equations into a single-variable equation, making it easier to handle and solve.

By diligently applying the substitution method, I can untangle the variables and find their values separately. Once I solve for one variable, I can substitute it back into the other equation to find the second variable’s value. Taking the time to understand the substitution method pays off greatly, as it gives me a systematic approach to solving equations that would otherwise seem complicated.

## Understanding the Substitution Method

When tackling systems of equations, I often choose the substitution method if one equation can be conveniently solved for one variable. This technique is particularly useful for systems that may not lend themselves nicely to the elimination method, especially when dealing with coefficients that don’t easily eliminate each other.

The substitution method involves three primary steps:

1. Solve one equation for one variable: I identify an equation where isolating a variable is most straightforward, normally due to coefficients of 1 or -1 that make the algebra simpler.

For example, if I have the system:

x + 3y = 6

2. 2x – y = 4

I might solve the first equation for $x$:

[ x = 6 – 3y ]

3. Substitute the expression into the other equation: Next, I substitute this expression into the other equation, which gives me an equation in one variable.

2(6 – 3y) – y = 4

4. Solve for the single variable: I simplify and solve the resulting equation for $y$:

12 – 6y – y = 4 ] [ -7y = -8 ] [ y = $\frac{8}{7}$

Now, having the value of $y$, I can find $x$ by substituting $y$ back into any of the original equations—let’s choose the first one:

x + 3$\left(\frac{8}{7}\right)$ = 6

x = 6 – $\frac{24}{7}$ ] [ x =$\frac{18}{7}$

I have found my solution to be $x = \frac{18}{7}$ and $y = \frac{8}{7}$. This means the system is consistent and the equations are independent; they intersect at a single point.

The substitution method is a reliable way to find the intersection point of two lines represented by the system of equations. It’s a testament to the beautiful intersection of algebra and geometry.

## Steps to Solve Systems of Equations

When I’m faced with a system of equations, the substitution method is my go-to technique for finding the unique solution. This approach involves several clear steps, and I’m happy to guide you through them. Just follow along!

1. Isolate a Variable: First, I pick one of the equations and solve for a single variable. Either x or y is fine, depending on which would simplify the most easily. For instance, if I have $2x + y = 7$, I’ll isolate y to get $y = 7 – 2x$.

2. Substitute the Expression: With my expression for one variable, I substitute it into the other equation in the system. If my second equation were $x – 2y = 6$, I replace y with the isolated expression to get $x – 2(7 – 2x) = 6$.

3. Solve for the Remaining Variable: After substitution, a single-variable equation remains, and I solve it. Simplifying the example above gives me $x – 14 + 4x = 6$, which simplifies further to $5x – 14 = 6$. Solving for x yields $x = 4$.

4. Substitute Back and Solve: Now that I have the value for x, I’ll use it to find y. Substituting x into $y = 7 – 2x$ gives me $y = 7 – 2(4)$, so $y = -1$.

5. Verify the Solution: Lastly, I always check my solution by plugging the values of x and y back into the original equations. A correct solution will satisfy both equations.

Here’s a quick table of the steps to keep track:

StepActionExample
1Isolate a Variable$y = 7 – 2x$
2Substitute the Expression$x – 2(7 – 2x) = 6$
3Solve for the Remaining Variable$x = 4$
4Substitute Back and Solve for the Other$y = -1$
5Check the Solution in Both Original EquationsVerify $(x,y)=(4,-1)$

Through substitution, solving for a variable, and checking the results, I can successfully solve the system of equations and find the solution that makes both equations true.

## Examples and Practice Problems

When I’m teaching algebra, one of my favorite methods to solve a system of equations is the substitution method. This involves replacing one variable with an equivalent expression from the other equation. Let me walk you through a couple of examples before you try some practice problems on your own.

Example 1:

Consider the system:

• (2x + 3y = 6)
• (x – y = 4)

First, I solve for (x) from the second equation: (x = y + 4). Next, I substitute (x) in the first equation:

(2(y + 4) + 3y = 6)
(2y + 8 + 3y = 6)
(5y + 8 = 6)
(5y = -2)
(y = -$\frac{2}{5}$)

Now, I substitute (y) in (x = y + 4):

(x = -$\frac{2}{5}$ + 4)
(x = $\frac{18}{5}$)

So the solution is (x = $\frac{18}{5}$), (y = -$\frac{2}{5}$).

Example 2:

Let’s solve the following system:

• (y = 2x + 3)
• (4x – y = 9)

Since (y) is already expressed in terms of (x) in the first equation, I substitute it directly into the second:

(4x – (2x + 3) = 9)
(2x – 3 = 9)
(2x = 12)
(x = 6)

Substituting (x = 6) back into the first equation gives me:

(y = 2(6) + 3)
(y = 15)

The system’s solution is (x = 6), (y = 15).

Now, it’s time for you to solve some on your own!

ProblemSystem of Equations
1(y = x – 1)
(3x + 2y = 12)
2(2x – 3y = -1)
(x = 4y + 5)
3(x + y = 5)
(x – 2y = 4)

Remember to simplify the systems as I did in the examples. This will help identify the solution without confusion. Keep practicing, and you’ll master the substitution method in no time!

## Conclusion

I’ve explored the substitution method as a means to solving systems of equations, which is a fundamental skill in algebra. The method works by solving one equation for one variable and then substituting the resulting expression into the other equation.

This reduces the system to a single equation in one variable, which can be solved directly. It’s especially convenient when one of the equations is already solved for a particular variable, making it easier to substitute.

For example, consider the system of equations given by ( y = 3x + 1 ) and ( 2x + 5y = 20 ). To solve this system, I’d substitute the expression for ( y ) from the first equation into the second, leading to a single variable equation that I can solve for ( x ). After finding ( x ), I would then plug it back into one of the original equations to solve for ( y ). The solution shows the point where the two lines represented by the equations would intersect on a graph.

Using this method effectively requires careful algebraic manipulation and attention to detail. Ensuring that your work is clear and methodical will minimize errors and help you understand the conceptual underpinnings of the technique. Remember, the key to mastery is consistent practice and application of the substitution method to a variety of problems.