Solving Linear Equations by Substitution

Today in class we learned how to solve equations by using substitution. This is when there is a problem that gives you two equations, sometimes in different forms. First you have to find what x= or what y=. Once you know that you take the unused equation and fill in either y or x. Then solve. Now you have the first number in the coordinate. To find the second you take the number you got for x or y and substitute into the same equation you used to find x or y. Then solve.

EXAMPLE

{3x+4y=9

{y=x-3 

3x+4(x-3)=9                 substitute x-3 for y

3x+4x-12=9                   distribute

7x-12=9                          add or subtract thing that are alike

7x=21                             +12 to opposite side to get x by itself

x=3                                 divide by 7 to get x=3

original equation

3x+4y=9        

3(3)+4y=9                      substitute

9+4y=9                           multiply(distribute)

4y=0                                -9 to get 4y by itself

y=0                                  divide by 4 to get y alone      

COORDINATE

(3,0)

Kathryn S.