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Completing the square help with coefficient other than 1 please help

WilliamSlim

(edited 9 years ago)

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Reply 1

TeeEm

Original post by WilliamSlim

http://i.imgur.com/NEwY2ZGh.jpg
Can someone please just go through the question and explain completing the square and what the hell I'm doing wrong, I've been doing this god forsaken question for over an hour now, I can do everything else in c1 except completing the square. I have no idea and considering just skipping learning completing the square as some people say you only need to divide the first 2 terms by the coefficient while others say you have to divide everything. I'm honestly about to punch my brother and break his jaw if he comes into my room again because doing this is making me so frustrated, i'm literally going to just go and drink as much alcohol as i can in 30 mintues if i can't do this and hope in my drunken state i pass out just to stop being so frustrated

firstly do you have an equation to solve by completing the square

or

complete the square in a quadratic expression

Reply 2

WilliamSlim

i should clarify, the question is 2x^2-7=4x and you have to find x

Reply 3

TenOfThem

Original post by WilliamSlim

i should clarify, the question is 2x^2-7=4x and you have to find x

2x^2 - 4x - 7 = 0

Start by dividing through by 2 and you don't have to worry about the leading coefficient

Reply 4

TeeEm

Original post by WilliamSlim

i should clarify, the question is 2x^2-7=4x and you have to find x

then write it in the usual order
then divide the equation by 2
then continue as per normal

Reply 5

TenOfThem

Original post by WilliamSlim

The issue with dividing everything or just the first 2 terms

You can always divide everything if you have an equation =0

However ... If you are just completing the square you cannot divide through ... Instead you take out a factor .... In this case you often leave the third term alone

Reply 6

WilliamSlim

http://i.imgur.com/Jbp0CsP.jpg
Yeah okay maths doesn't work

Reply 7

TenOfThem

Original post by WilliamSlim

http://i.imgur.com/Jbp0CsP.jpg
Yeah okay maths doesn't work

That was fine ... What is the problem

Did you want

\dfrac{2\pm3\sqrt{2}}{2}

(edited 9 years ago)

Reply 8

WilliamSlim

Original post by TenOfThem

That was fine ... What is the problem

Did you want

\dfrac{2+-3\sqrt2}{2}

No, i don't understand at all why you are dividng it by 2 at the end, or where on earth you got 2 before the +-sqrt

Reply 9

TenOfThem

Original post by WilliamSlim

No, i don't understand at all why you are dividng it by 2 at the end, or where on earth you got 2 before the +-sqrt

Your answer was fine ... What is the issue

Reply 10

WilliamSlim

http://i.imgur.com/83Xas9K.jpg
Question 6, my answer is wrong, it's also different from your answer.

You got 1+-(3sqrt2/2) I have no idea at all why you multiply the 3 by the sqrt2 and then why you divide it by 2

(edited 9 years ago)

Reply 11

TenOfThem

Original post by WilliamSlim

http://i.imgur.com/83Xas9K.jpg
Question 6, my answer is wrong, it's also different from your answer.

Your answer is the same as my answer

And both of our answers are the same as the one in your link

Reply 12

WilliamSlim

You got 1+-(3sqrt2/2) and i got 1+-3/sqrt2 I have no idea at all why you multiply the 3 by the sqrt2 and then why you divide it by 2

Reply 13

WilliamSlim

Oh my f****** i just relaised they rationalsied it, why why i spent 2 hours trying to figure out what went wrong

DO I ALWAYS HAVE TO RATIONALISE IT? IT DIDNT EVEN ASK TO SIMPLIFY!!!!!

Reply 14

TenOfThem

Original post by WilliamSlim

Oh my f****** i just relaised they rationalsied it, why why i spent 2 hours trying to figure out what went wrong

DO I ALWAYS HAVE TO RATIONALISE IT? IT DIDNT EVEN ASK TO SIMPLIFY!!!!!

The answers in the back if the book will always have rationalised denominators

Rationalised denominators are the accepted form

You should not lose marks in an exam if you don't .... But I would suggest that you do

Reply 15

WilliamSlim

Original post by TenOfThem

Okay i am still confused on when i should only factor out the coeffcieint from the first two numbers and when i should factor it out from the whole sum. If i am asked to just complete the square and present it in a form i factor out only the first two terms right? And if it it is asking me to solve for x i'd factor it out from everything right?

Reply 16

TenOfThem

Original post by WilliamSlim

If you wish .... You could ALWAYS factor for the first 2,and leave the other alone

Let me do a couple of examples and add them for you

Reply 17

WilliamSlim

Okay thanks man, sorry but i have my C1 mck coming up on March and i can doe evrything except complete the square

(edited 9 years ago)

Reply 18

TenOfThem

Original post by WilliamSlim

Hope these help

Reply 19

WilliamSlim

Original post by TenOfThem

Hope these help

Woah huge help dude thanks, but what happens to the 5 in the red/second question? I swear my teacher said we couldn't just make the number disappear when we are factoring it.