Simultaneous equations require algebraic skills to find the values of letters within two or more equations. They are called simultaneous equations because the equations are solved at the same time.

Equations that have more than one unknown can have an infinite number of solutions. For example, could be solved by:

and

and

and

To be able to solve an equation like this, another equation needs to be used alongside it. That way it is possible to find the only pair of values that solve both equations at the same time. These are known as simultaneous equations.

An example of this is:

and

The unknowns of and have the same value in both equations. This fact can be used to help solve the two simultaneous equations at the same time and find the values of and .

Solving simultaneous equations by elimination

The most common method for solving simultaneous equations is the elimination method which means one of the unknowns will be removed from each equation. The remaining unknown can then be calculated. This can be done if the coefficient of one of the letters is the same, regardless of sign.

Example

Solve the following simultaneous equations:

First, identify which unknown has the same coefficient. In this example this is the letter , which has a coefficient of 1 in each equation.

Either add or subtract the two equations from each other to eliminate the letter . In this example the equations will need to be subtracted from each other as .

If the equations were added together, then , and so the letter would not be eliminated.

The value of can now be substituted into either equation to find the value of .

Substitute into either or .

when

Substitute :

Find the value of using inverse operations to solve equations. The inverse of adding 9 is subtracting 9, so subtract 9 from each side:

Check the answers by substituting both values into the other original equation. If the equation balances, then the answers are correct: