## Lesson 13: Simultaneous Linear Equations (I)

February 28, 2011

Vocabulary on simultaneous linear equations

### 1. Linear equations

A linear equation in two unknowns is an equation of first degree with two unknowns.

Any linear equation can be expressed in its standard form

ax + by = c

where a, b and c are known numbers.

A solution of a linear equation in two unknowns is a pair of values that verify the equation (they make it true)

A linear equation in two unknowns has infinite solutions.

We call system (or pair) of linear equations or simultaneous equations in two unknowns to a collection of such equations.

ax + by = c

a’x + b’y = c’

A solution of the system is any pair of numbers that satisfies them simultaneously.

Solving a pair of simultaneous equations is to look for the values of the unknowns which make all the equations true simultaneously

### 2. Methods of solving simultaneous equations

We now focus on various methods of solving simultaneous equations. All these methods have the same philosophy: to obtain from the two equations another equation only in one unknown. Once we solve it is easy to determine the value of the other unknown.

### 2.1. Substitution method

Here it is the method of substitution:

• Solve one of the equations for one unknown in terms of the other.
• Then, substitute that in the other equation.
• That will yield one equation in one unknown, which we can solve.
• We obtain the other unknown substituting in any equation of the system

In this page you can introduce any system of linear equations, solve it by yourself and find a complete explanation about how to do it by using substitution method. Unfortunately, this algebra solver don’t use elimination method.

Here you are a video on substitution method.

### 2.2. Equalizing Method

This method is not explained in Anglo-Saxon countries, probably because it is a slight variation of substitution method.

Here it is the method of “Equalizing”:

• Solve both equations for the same unknown in terms of the other.
• Then, equalize the expressions in terms of the other.
• That will yield one equation in one unknown, which we can solve.
• We obtain the other unknown substituting in any equation of the system

Sorry, I couldn’t find a web page where the equalizing method were explained and that had interactive exercises.

### 2.3. Elimination method

This method is also named addition method.

As the name suggest, this method tries to eliminate variables until there’s only 1 variable left by adding the equations.

Here it is a general strategy for solving simultaneous equations:

• Look at the equations and try to find 2 equations which has the same coefficient (plus or minus) for the same variables.
• When the coefficients don’t have the same coefficients (plus or minus), multiply the equations so that the coefficients of that unknown are the same
• Add or subtract  the equations vertically, and that unknown will cancel(we eliminate the unknown)
• We will then have one equation in one unknown, which we can solve.
• To solve for y, the other unknown substitute the value of x in one of the original equations.

This page is highly recommendable because you can find several videos on solving simultaneous equations by using elimination method, but they have got a higher level. The video below is elemental but good to improve your listening

In the page I am going to recommend you, you have interactive exercises on solving simultaneous equations by using elimination method. It has a difficulty because they always subtract instead of add up as we do.

Take into account that if you subtract an equation from other you have to change every sign in the LHS and in the RHS to subtract correcly.

Go down the page to find the exercise. Get an example and try it by yourself. If you have got any difficulty, click on the buttons below. Finally, use only example 1. Example 2 is harder to understand for you.

1. Hi mónica…!:) I`ve found a very good maths webpage, it explains you the things very well (but in Spanish, what a pity…) that page’s name is:
http://www.vitutor.com/index.html

2. sorry I was wrong… if you put the uk flag the page is in English…

3. A set of independent equations in two or more variables is called a simultaneous equation. To find the value(s) of the variables for which the set of equation holds is called solving the equation. There are different methods used to solve simultaneous equations. In this article, we will learn how to solve the simultaneous equations by Substitution and Elimination methods with examples.

http://math.tutorvista.com/algebra/simultaneous-equations.html

4. The traduction of the video:
-I.N.T.E.G.E.R.
-Serás una estrella.
-Esta es la mecánica de los números que somos.
-Positivo,Negativo y….. ceroo.
-Positivo,Negativo y….. ceroo.
-Ya que lo sabes se un héroe de las Mates.
-Sumando,restando usa la línea numérica y verás que guay.
-Los aprenderás tan rápido que tu profesora podrá flipar, y flpar…. y flipar…. y flipar….. y flipar….
-Multiplicando o dividiendo usa las reglas.

5. I.N.T.E.G.E.R.
-Serás una estrella.
-Integer.
-Esta es la mecánica de los números que somos.
-Positivo.
-Negativo.
-Y… cero.
-Positivo, negativo y … cero.
-Ya que lo sabes sé un héroe de las mates.
-Sumando y restando usa la recta numérica y verás que guay
Los aprenderás tan rápido que tu profe querrá flipar…flipar…
-Multiplicando o dividiendo usa las reglas…