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Homework Help: System of Equations and Inequalities

  1. Jun 2, 2010 #1
    1. The problem statement, all variables and given/known data

    Solve each sysytem usiing any method you wish:
    x2-y2=21
    x+y=7


    2. Relevant equations

    x2-y2=21
    x+y=7


    3. The attempt at a solution

    I chose to do the substitution method.

    so i change equation 2 to this: y=7-x and substitute it into the other equation in place of y.

    so now equation 1 is this x2-(7-x)2=21

    so i foil out that part to get x2-(49-14x+x2)=21

    now this is where im stuck im not sure what to doo becuase im not sure if the - outside the paranthesis turns everything inside them to the oposite sign? or what?
     
  2. jcsd
  3. Jun 2, 2010 #2

    rock.freak667

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    Right the -ve or '-1' in front of the bracket, multiplies everything inside by -1. So expand out.
     
  4. Jun 2, 2010 #3
    so i would get x2-49+14x-x2=21?
     
  5. Jun 2, 2010 #4

    rock.freak667

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    Yes.
     
  6. Jun 2, 2010 #5
    so would equal -2 is what i got then i sustituted that into y=7-(-2) to get y =9.

    those would be my anwser correct?
     
  7. Jun 2, 2010 #6

    rock.freak667

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    if you sub that into the first equation you do not get 21, check it over.
     
  8. Jun 2, 2010 #7
    yeah but it work for the 2nd equation what did i do wrong did i skip something?
     
  9. Jun 2, 2010 #8
    woops haha i was doing something wrong with my math i found x actally equal 5 and y= 2
     
  10. Jun 2, 2010 #9

    rock.freak667

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    That should be right now
     
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