Completing the square please help!!!!

Original post by Tweaky

It should be (x-2.25)^2 - 5.0625 = 3.5.

You have written it as +ve.

Reply 2

Original post by raheem94

It should be (x-2.25)^2 - 5.0625 = 3.5.

You have written it as +ve.

No wait...

(x-2.25)(x-2.25) = x^2 -4.5x then (-2.25 x -2.25 is positive) So it is positive...?

(edited 11 years ago)

Reply 3

steve2005

Original post by Tweaky

Hey this is the question and this is what I've done so far:

Complete the square and find solution to x.

- 2x^2 - 9x - 7 = 0

The obvious first thing to do, is to multiply throughout by minus one.

Reply 4

Original post by steve2005

The obvious first thing to do, is to multiply throughout by minus one.

It isn't a negative sign, he is just putting them in front of all his lines.

Reply 5

Original post by steve2005

The obvious first thing to do, is to multiply throughout by minus one.

its not " -2x^2"

The - was a bullet point. If that's not what you meant, could you elaborate because I don't understand. Cheers man :smile:

Reply 6

Original post by Tweaky

its not " -2x^2"

The - was a bullet point. If that's not what you meant, could you elaborate because I don't understand. Cheers man :smile:

Poor choice of "bullet"

Reply 7

Original post by TenOfThem

Poor choice of "bullet"

Great, but now you know could you help me please xx

Reply 8

Original post by Tweaky

No wait...

(x-2.25)(x-2.25) = x^2 -4.5x then (-2.25 x -2.25 is positive) So it is positive...?

x^2 - 4.5x = 3.5

\displaystyle x^2 - \frac92 x = \frac72 \implies \left(x- \frac94 \right)^2 - \left( \frac94 \right)^2 = \frac72

\displaystyle \left(x- \frac94 \right)^2 = x^2 - \frac92 x + \left( \frac94 \right)^2

So it should be negative,

Do you get it?

Reply 9

Original post by Tweaky

Great, but now you know could you help me please xx

No need for me to do so

Raheem has told you where you went wrong but you chose to argue with him

Reply 10

Intriguing Alias

Original post by Tweaky

You want x^2 - 4.5x on the LHS. So this is the same as writing (x-2.25)^2 + an adjustment. (x-2.25)^2 = x^ - 4.5x + 5.0625 so our 'adjustment' must be -5.0625 to cancel it out.

Reply 11

Original post by TenOfThem

No need for me to do so

Raheem has told you where you went wrong but you chose to argue with him

I just didn't understand.

Reply 12

OMGWTFBBQ

2x^2 - 9x - 7 = 0
x^2 - (9/2)x - (7/2) = 0
(x - 9/4)^2 - (9/4)^2 - (7/2) = 0
(x - 9/4)^2 - 81/16 - 7/2 = 0
(x - 9/4)^2 - 81/16 - 55/16 = 0
(x - 9/4)^2 - 136/16 = 0
(x - 9/4)^2 - 17/2 = 0
(x - 9/4)^2 = 17/2

Can you go from here?

PS. Use fractions where you can. It's better maths.

(edited 11 years ago)

Reply 13

Original post by Tweaky

I just didn't understand.

Then I suggest that you go and look at your notes on "completing the square" as you seem to have a basic mis-understanding of the process

Reply 14

Original post by Tweaky

I just didn't understand.

Go to this link to learn this.

Reply 15

Original post by hassi94

You want x^2 - 4.5x on the LHS. So this is the same as writing (x-2.25)^2 + an adjustment. (x-2.25)^2 = x^ - 4.5x + 5.0625 so our 'adjustment' must be -5.0625 to cancel it out.

Sweet, yeah I get it now. Was a small error, cheers. And thanks Raheem. To the other two users, arrogance won't get you far in life.

Reply 16

Original post by Tweaky

I didn't realize it was a crime to make a mistake and not understand someone straight away.

I simply pointed out the bullet was a bad choice

Your response was not polite ... and I pointed out that you already had the answer

I think you need to read responses to the question, think about them, and absorb the maths advice instead of jumping in and saying "no"

I will also re-iterate that you have demonstrated a lack of understanding re this topic and one question on here is not enough to redress that

And you gave me a neg .... I pointed out that someone had told you the answer and you negged me ... seriously?

(edited 11 years ago)

Reply 17

Original post by TenOfThem

Hey I'm sorry for being rude to you - I was annoyed with steve's comment as it was rude. So I was quick to jump to conclusions when reading your comments. So I'm sorry for that.

But I understand where I wen't wrong on that and have just finished 5 other completing the square questions, so I understand what I did wrong - Just made small mistake. But I apologize to you.

And thanks for everyone who answered the question.

Reply 18

Original post by Tweaky

But I understand where I wen't wrong on that and have just finished 5 other completing the square questions, so I understand what I did wrong - Just made small mistake.

Fair enough

The error you made is the single most common mis-understanding in this topic so do make sure that you understand why and that you are not just copying Raheem's example

Reply 19