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Maths Difficulty.....Please Help Somebody :)

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theone

Sure, all quadratics can be written in the form x^2 +bx + c.

Now we can note by multiplying out that this is in fact equal to (x+(b/2))^2 + (c - b^2/4).

This procedure of getting the second equation from the first is called completing the square.

By extending this you can work out the more general formula for the solution of a quadratic.

Theone could you just confirm that:

(x) = 3[(x-1r1r3)^2+10r2r3] in completed square form

is the right answer.

Reply 21

cimmie

just draw it and look :biggrin:

Reply 22

++Hex++

clever_lad

i am correct, you dont ahve a clue what you are talking about

No, you've forgotten to divide the 8x (the 4) by 3 when you took it out... it should be 4/3.

Reply 23

theone

Bhaal85

Theone could you just confirm that:

(x) = 3[(x-1r1r3)^2+10r2r3] in completed square form

is the right answer.

Well, it ain't going to be since you have - 4/3 and the original equation has a positive term in x. :smile:

I get f(x) = 3((x+4/3)^2 - 16/9) which gives x = -4/3 as the max.

You get the same by differentiation :smile:

Reply 24

Bhaal85

++Hex++

No, you've forgotten to divide the 8x (the 4) by 3 when you took it out... it should be 4/3.

So I am right. And that arse was just being rude and arrogant?

Reply 25

Bhaal85

theone

Well, it ain't going to be since you have - 4/3 and the original equation has a positive term in x. :smile:

I get f(x) = 3((x+4/3)^2 - 16/9) which gives x = -4/3 as the max.

You get the same by differentiation :smile:

*goes and gets pen and paper*

Reply 26

theone

Bhaal85

So I am right. And that arse was just being rude and arrogant?

Who's the arse?

Reply 27

++Hex++

Bhaal85

So I am right. And that arse was just being rude and arrogant?

No need for that. Everyone makes mistakes. Especially where maths is involved...

Reply 28

theone

Sorry i've made and error it should read

3((x+4/3)^2-32/9) i think.

Reply 29

Bhaal85

theone

Who's the arse?

notsoclever_lad

Yeah your right dy/dx=0 at x=-8/6. ben revising for Discrete for 4 weeks. lol. Not arsed, but that arse was wrong.

Reply 30

Bhaal85

clever_lad

i am correct, you dont ahve a clue what you are talking about

Thats what the arse wrote, to m that is being rude and arrogant, considering I was helping him.

i was right!!

Bhaal85

Thats what the arse wrote, to m that is being rude and arrogant, considering I was helping him.

Oh, ok, thought that you were having a go at me, no offence :smile:

Reply 33

Bhaal85

theone

Sorry i've made and error it should read

3((x+4/3)^2-32/9) i think.

10r2r3 not 32/9

Reply 34

cimmie

i was right!!

hmmpf whats about funny pictures?? :frown:

Reply 35

Bhaal85

cimmie

i was right!!

how are you right??? Hmm...you must be notsoclever_lad then???

Reply 36

theone

Bhaal85

10r2r3 not 32/9

No i don't think.... since we want to complete the square of x^2 +8x/3 +16/3 = (x+4/3)^2 + 16/3 - 16/9.

And 16/3 - 16/9 = 32/9

Reply 37

clever_lad

oops, sorry Bhall, i didnt complete the square correctly. i am really sorry mate, i take my comment back *shrieks with embarrasment*........if you dont mind mate, please could you go through the process of completing the square for this equation, then showing me what x value will give the minimum value for the parabola.....please mate...once again, sorry. *shudders*

Reply 38

cimmie

x=-4/3

f'(x):6x+8
=>6x+8=0
<=>x=-4/3

sorry its just guessed :biggrin:

i had it first... :biggrin:

but forget this stupid thing! Its just confusing me! :wink:

Reply 39

Bhaal85

theone

No i don't think.... since we want to complete the square of x^2 +8x/3 +16/3 = (x+4/3)^2 + 16/3 - 16/9.

And 16/3 - 16/9 = 32/9

dy/dx = 6x+8 right.

set to equal 0 = 6x+8=0
rearrange to make x subject you get: x=-8/6

sub that into orignal equation 3x^2+8x+16 = 10r2r3. (ten and two thirds)

:biggrin: