Helping with homework!  TOPIC_SOLVED

Quadratic equations and inequalities, variation equations, function notation, systems of equations, etc.

Helping with homework!

Postby R515 on Sun Oct 20, 2013 10:07 am

Please go easy on me as it's 35yrs since I did this at school! I'm helping my daughter and the equation is

x^2 - y^2 =12 where y = x-2 .

By substitution I've worked the answer out as x=4 but none of the methodology (e.g. factorisation or quadratic formula) in her notes or the text books give me the right answer. Through factorisation, x = -2 which is wrong, any one able to help as this driving me nuts!!

Rgds, Mark
R515
 
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Re: Helping with homework!

Postby buddy on Sun Oct 20, 2013 12:49 pm

R515 wrote:the equation is

x^2 - y^2 =12 where y = x-2 .

What are you supposed to do with this? Are you solving a system of equations?

R515 wrote:none of the methodology (e.g. factorisation or quadratic formula) in her notes or the text books give me the right answer.

Please show what you did so we can see what's going wrong.

R515 wrote:Through factorisation, x = -2 which is wrong

How did you get this by factoring??

Substitution is what they show here for doing this.

x^2 - (x - 2)^2 = 12
x^2 - (x^2 - 4x + 4) = 12
etc

What did you get from this?
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Re: Helping with homework!

Postby R515 on Sun Oct 20, 2013 1:43 pm

Thanks for the reply, the question is to find the value of x and y, this is how it went using factorisation as my daughter has been taught:

x^2 - (x-2)^2 = 12
x^2 - (x-2)(x-2) = 12
x^2 -x^2 -4x +4 = 12
x^2 -x^2 -4x = 8
so -4x =8
which gives x= -2 but substituting -2 for x will not give the correct answer. Where are we going wrong?

Thx.
R515
 
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Re: Helping with homework!

Postby buddy on Sun Oct 20, 2013 3:42 pm

R515 wrote:x^2 - (x-2)^2 = 12
x^2 - (x-2)(x-2) = 12
x^2 -x^2 -4x +4 = 12

Try doing grouping like I showed so you don't forget the minus:

x^2 - (x^2 - 4x + 4) = 12
x^2 - x^2 + 4x - 4 = 12

when you do the signs right the rest should come out right too.
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Re: Helping with homework!

Postby R515 on Sun Oct 20, 2013 5:01 pm

Thanks for taking the time to help, think I've nearly got it, is it the effect of removing the bracket on

x^2 -(x^2 +4x -4) =12

that turns +4x into -4x and -4 into +4 then as this is what makes it work out right with x=4 ?

Thanks Mark
R515
 
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Re: Helping with homework!  TOPIC_SOLVED

Postby buddy on Sun Oct 20, 2013 9:15 pm

R515 wrote:is it the effect of removing the bracket on

x^2 -(x^2 +4x -4) =12

that turns +4x into -4x and -4 into +4 then as this is what makes it work out right with x=4 ?

Yeah it's the negatives. You can learn about doing negatives with brackets here. It's kind of important.
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Re: Helping with homework!

Postby R515 on Mon Oct 21, 2013 3:20 pm

Thanks for the help, it finally came to me yesterday evening how the (-) outside the bracket would influence what happens to the numbers inside when the brackets are removed, appreciate the help given,

Mark
R515
 
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