Completing the Square (Cant get correct answer)

x² + 3x + 1 = 0

Solve this equation by completing the square:

I cant write the answer I have as the settings on this site aren't giving me the tools to do so.

Reply 1

alow

Original post by ero

x² + 3x + 1 = 0

Solve this equation by completing the square:

I cant write the answer I have as the settings on this site aren't giving me the tools to do so.

Sure. Whatever you say.

Reply 2

TeeEm

Original post by ero

x² + 3x + 1 = 0

Solve this equation by completing the square:

I cant write the answer I have as the settings on this site aren't giving me the tools to do so.

post the workings as a picture

Reply 3

ero

Really messy workings but at least it feels good to shut alow up.

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Reply 4

davros

Original post by ero

Really messy workings but at least it feels good to shut alow up.

When you worked out the square of the bracket (x + 3/2), you should have had a term 9/4 which you would need to subtract. What has happened to this?

Reply 5

TeeEm

Original post by ero

Really messy workings but at least it feels good to shut alow up.

from line 3 to line 4 I think it is totally incorrect.

What are you doing between these 2 lines?

Reply 6

TenOfThem

Original post by ero

I cant write the answer I have as the settings on this site aren't giving me the tools to do so.

http://www.thestudentroom.co.uk/wiki/LaTex

Reply 7

ero

So what I have worked out that it is meant (as was mentioned) to include 9/4 as I forgot to square the b coefficient as it is meant to be (b/2)^2 so therefore 3x as by would equal (3/2)^2. I understand this and just forgot it but now I am left with the following: (Considering on the right hand side the real answer is all over 2, how does 5/4 and 3/2 all randomly turn to 2?

(edited 9 years ago)

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Reply 8

TeeEm

Original post by ero

gosh I am too old to do a hand stance ...

Reply 9

TenOfThem

Original post by ero

You have

\sqrt(\frac{5}{4}) = \dfrac{\sqrt5}{\sqrt4} = \dfrac{\sqrt5}{2}

Reply 10

DFranklin

Original post by ero

x² + 3x + 1 = 0

Solve this equation by completing the square:

I cant write the answer I have as the settings on this site aren't giving me the tools to do so.

I know you've already had the answer, but regarding "tools".

First, if you're going to post here a lot, it's really worth learning to use LaTeX; there's a guide (which TenOfThem has already linked): http://www.thestudentroom.co.uk/wiki/LaTex.

Even if you can't use LaTeX, it's perfectly possible to write out your workings for something like this using plain text. E.g.

x^2 + 5x + 3 = 0, so
x^2 + 5x = -3, so
(x+ 5/2)^2 - (5/2)^2 = -3, so
(x+ 5/2)^2 = -3 + 25 / 4 = 13 / 4, so
x + 5/2 = +/- sqrt(13/4), so
x = -5/2 +/- sqrt(13/4), so
x = (-5 +/- sqrt(13)) / 2

Reply 11

ero

Original post by TenOfThem

You have

\sqrt(\frac{5}{4}) = \dfrac{\sqrt5}{\sqrt4} = \dfrac{\sqrt5}{2}

Ahh I see, thankyou for the help

Reply 12

ero

Original post by DFranklin

Thankyou, it will take some time to get used to, also how come you minus the (5/2)^2 to the left hand side rather than add it to the left hand and right hand side.

Reply 13

DFranklin

Original post by ero

Thankyou, it will take some time to get used to, also how come you minus the (5/2)^2 to the left hand side rather than add it to the left hand and right hand side.

Because x^2 + 5x = (x+5/2)^2 - (5/2)^2