Solve for x and y using the addition method. A) 3x-y= -8 -9x+2y=18      

3 Answers | Add Yours

durbanville's profile pic

durbanville | High School Teacher | (Level 2) Educator Emeritus

Posted on

  1. `3x-y = -8`
  2. `-9x+2y=18`

We need to eliminate either x or y to enable us to solve so multiply  1. by 2 so that we will have -2y:

`therefore 3. (times 2) 6x - 2y = -16`  now add 2. to that

3. + 2. = 6x + (- 9x) and  2y + (- 2y) and that equals  -16 + 18 

`therefore -3x +0y= 2`

`therefore -3x=2`

`therefore x=2/-3`

Now substitute into one of your earlier equations (any one) to find y:

`3x - y = -8`

`therefore 3(-2/3) - y = -8`

`therefore-2 - y = -8`

`therefore -y= -8+2`

`therefore - y= -6`

`therefore y=6`

`(-2/3; 6)`

Therefore x= -2/3 and y = 6

Sources:
Wiggin42's profile pic

Wiggin42 | Student, Undergraduate | (Level 2) Valedictorian

Posted on


(1) 3x - y = -8
(2) -9x + 2y = 18

(1) x 2:

6x - 2y = -16

Add this to equation (2)
-3x = 2

Now its single variable and you can solve for x. Plug this back into one of the original equations and solve for y.

oldnick's profile pic

oldnick | (Level 1) Valedictorian

Posted on

`3x-y=-8`

`-9x+2y=``18`

Multiply first for 3:

`9x-3y=-24`

add first to second:

`-9x+2y+9x-3y=18-24`

`-y=-6`

`y=6`   

From first line:

`3x-6=-8`

`3x=-2`     `x=-2/3`

 

We’ve answered 318,989 questions. We can answer yours, too.

Ask a question