System of Equations and Inequalities

1. Jun 2, 2010

Everstar

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. Jun 2, 2010

rock.freak667

Right the -ve or '-1' in front of the bracket, multiplies everything inside by -1. So expand out.

3. Jun 2, 2010

Everstar

so i would get x2-49+14x-x2=21?

4. Jun 2, 2010

Yes.

5. Jun 2, 2010

Everstar

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?

6. Jun 2, 2010

rock.freak667

if you sub that into the first equation you do not get 21, check it over.

7. Jun 2, 2010

Everstar

yeah but it work for the 2nd equation what did i do wrong did i skip something?

8. Jun 2, 2010

Everstar

woops haha i was doing something wrong with my math i found x actally equal 5 and y= 2

9. Jun 2, 2010

rock.freak667

That should be right now