Hud5afaa
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#1
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#1
Hey guys
so there's an equation to solve, that is meant to be done by completing the square.

2x^2 - 8x + 7 = 0

So, I did it as:

2(x^2 - 4x) + 7 = 0
.
2((x-2)^2 -4) + 7 = 0

then

2(x-2)^2 + 3 = 0

then (x-2)^2 = -3/2 , then (x-2) = root -3/2 etc etc


However the answer in the solution bank says the answer becomes
(x-2)^2 = 1/2, then (x-2) = root 1/2 etc etc

Why is my method wrong?
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Muttley79
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#2
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(Original post by Hud5afaa)
Hey guys
so there's an equation to solve, that is meant to be done by completing the square.

2x^2 - 8x + 7 = 0

So, I did it as:

2(x^2 - 4x) + 7 = 0
.
2((x-2)^2 -4) + 7 = 0

then

2(x-2)^2 + 3 = 0

then (x-2)^2 = -3/2 , then (x-2) = root -3/2 etc etc


However the answer in the solution bank says the answer becomes
(x-2)^2 = 1/2, then (x-2) = root 1/2 etc etc

Why is my method wrong?
2((x-2)^2 -4) + 7 = 0

then

2(x-2)^2 + 3 = 0 this isn't correct - if you expand the above you get 2(x - 2)^2 - 8 + 7 = 0 I think you forgot to multiply the -4
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RDKGames
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#3
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(Original post by Hud5afaa)
2((x-2)^2 -4) + 7 = 0
then
2(x-2)^2 + 3 = 0
You didn't multiply the -4 by 2.
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Hud5afaa
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#4
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(Original post by RDKGames)
You didn't multiply the -4 by 2.
Ahh you're spot on man very silly mistake.
However when I multiply the -4 and 2 and do the same method, I get (x-2)^2 = -1/2 , and their answer is (x-2)^2 = 1/2
I know if you squareroot -1/2 you get the same answer as if you squareroot 1/2, so is it still correct?
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RDKGames
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#5
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(Original post by Hud5afaa)
Ahh you're spot on man very silly mistake.
However when I multiply the -4 and 2 and do the same method, I get (x-2)^2 = -1/2 ,
Not quite. Before you move anything to the RHS, you should be careful because -8 + 7 = -1.

Hence when you move it to the RHS it becomes +1.

I know if you squareroot -1/2 you get the same answer as if you squareroot 1/2, so is it still correct?
Incorrect -- you cannot square root a negative number (without further knowledge).
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Hud5afaa
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#6
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(Original post by RDKGames)
Not quite. Before you move anything to the RHS, you should be careful because -8 + 7 = -1.

Hence when you move it to the RHS it becomes +1.



Incorrect -- you cannot square root a negative number (without further knowledge).
Thanks man. Really need to cut out these silly mistakes
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