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Solve this system of equations.  {2x+y+z=1 {-x-3y+z=9 {x-y+2z=7 (All in one bracket...

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kristenmarieb... | Student, Grade 10 | Valedictorian

Posted February 24, 2013 at 4:07 PM via web

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Solve this system of equations. 

{2x+y+z=1

{-x-3y+z=9

{x-y+2z=7

(All in one bracket sorry) 

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embizze | High School Teacher | (Level 1) Educator Emeritus

Posted February 24, 2013 at 4:54 PM (Answer #1)

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Solve the following system:` {[2x+y+z=1],[-x-3y+z=9],[x-y-2z=7]}`

The are a number of ways to solve this using matrices. We can also use substitution or linear combinations.

(1) Substitution: Solve one of the equations for one of the variables. We will solve the first equation for z:

z=1-2x-y Now substitute this expression in place of z in the last two equations:

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

x-y+2(1-2x-y)=7 
-------------------------- Eliminate the parantheses and collect like terms:

-3x-4y=8
-3x-3y=5     It is easier to use combinations here: subtract the first equation from the second to get:

`y=-3`

Then `-3x+12=8==> -3x=-4 ==>x=4/3`

Now ` ``z=1-2x-y==>z=1-2(4/3)-(-3)==>z=1-8/3+3==>z=4/3`

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The solution is `x=4/3,y=-3,z=4/3`

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(2) Using linear combinations:

2x+y+z=1   I
-x-3y+z=9  II
x-y+2z=7   III

Take I-II ==> 3x+4y=-8  

Take III-2I==> -3x-3y=5

Which is the same 2 equations in x and y we had above.

 

 

 

 

 

 

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