Simultaneous Equations

We can solve simulataneous equations by substitution or elimination.

Elimination


34x - y = 2

18x + 14y = 2

Lets convert the coefficients to whole numbers - integers by multiplying each equation by 8

6x - 8y = 16

x + 2y = 16

We now want either the coefficient of x or coefficient of y to be the same. Lets choose the coefficient of x to be both 6. We will need to multiply the secondary equation by 6

6x - 8y = 16

6x + 12y = 96

Now we can subtract the first equation from the second

20y = 80

We just have one equation with one unknown variable. We can see that y = 4

Finally we need to find x so choose any of the original equations. I will choose the second equation since the coefficients are small numbers

x + 2y = 16

Finally substitute y = 4 to find the x value

x = 8

The final solution is therefore x = 8 and y = 4

ALTERNATIVELY

6x - 8y = 16

x + 2y = 16

You may have prefered to have the y coefficients the same so we will multiply the second equation by 4

6x - 8y = 16

4x + 8y = 64

Since the signs are different, if we subtract one equation from the other we will still have two unknown variables. In this case we should add the equations.

10x = 80

Solving this equation we can see that x is 8. We can also substitute the x = 8 into any of the original equations and will find again that y = 4

The final solution is therefore x = 8 and y = 4

Solving by substitution


6x - 8y = 16

x + 2y = 16

First we can find an expression for x in terms (or y in terms of x). Then substitution that expression in the other equation. I will express x in terms of y since there are less steps.

x = 16 - 2y

Now I will substitute this expression into the first equation.

6(16-2y) - 8y = 16

We can expand and simplify

96 - 20y = 16

Solving this equation we can see that y = 4

Finally substitute y =4 into any of the original equations to find x. I am going to choose the second equation because the coefficients are smaller

x + 2(4) = 16

We see that x = 8 and also y =4

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