The Difference of Two Cubes (1 Viewer)

Kyroth***

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That question... grrrrrrrrrrr
I know difference of two squares, easy! But cubes? I spect 5min choosing random factorisations and expanding them out, to no avail!
*screems in frustration*
Can someone, please, tell me what x3+27 is factorised?
 

sikeveo

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(a^3 + b ^3) = (a + b)(a^2 + ab + b^2)

Go from there. Btw dont u mean x3-27?
 

b-rex

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there is a formula.....

eg
(a3 - b3) = (a - b) (a2 + ab + b2)

sooooo

x3-27 is
(x - 3) (x2 + 3x + 9)

i got the signs mixed up in the question and got it wrong away :(
 
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the question was x^3-27
Answer: (x-3)(x^2 + 3x + 9)..
I got it wrong too.. purely out of exam stress, because i know how to do it but i remember writing 6x instead of 3x.. im so upset, i cant afford to lose any stupid marks like that.. i did so crap on the other parts..
 

rachey_poo

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the question in the paper was actually x^3 - 27, not +
factorising cubics:
a^3 - b^3 = (a-b)(a^2 + ab + b^2)

a^3 + b^3 = (a+b)(a^2 - ab + b^2)

hope that helps...!
 

littlebinzy

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Kyroth*** said:
That question... grrrrrrrrrrr
I know difference of two squares, easy! But cubes? I spect 5min choosing random factorisations and expanding them out, to no avail!
*screems in frustration*
Can someone, please, tell me what x3+27 is factorised?
I lost marks for this in a trial somewhere so I remembered it...

sum of two cubes is (x+y)(x2 -xy+y2)
difference of two cubes is (x-y)(x2+xy+y2)

hence x3-27 = (x-3)(x2+3x+9)

I laughed to myself because just before we went in, a friend and I were looking at my formula sheet and joking about how we hoped that diff of cubes wasn't in it because we didn't know it very well...
 

Yaz88

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hahaha i rote that then i crossed it out n rote 6x instead of 3x
 

goan_crazy

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I can normally never do these
But I spent a while on it until I got it
and I got (x-3)(x^2 + 3x + 9) :)
Yay!

question 1 may have been good but it went downhill from there
I didnt even get 1f :( parabola q
 

0433

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goan_crazy said:
question 1 may have been good but it went downhill from there
I didnt even get 1f :( parabola q
hmmm that was focus is at (0, 2), right?
 

seremify007

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Kyroth*** said:
That question... grrrrrrrrrrr
I know difference of two squares, easy! But cubes? I spect 5min choosing random factorisations and expanding them out, to no avail!
*screems in frustration*
Can someone, please, tell me what x3+27 is factorised?
ROFL ME TOO! I did that... I wasn't sure if it was - + - or - + + ~ I went through and did it using A's and B's to figure it out.

I also forgot my Sum formula (for the repayments/geometric series Q's) so I spent about 10mins trying to remember- I kept trying to use the formula I remembered with the GP "3, 6, 9" until I realised- wait a minute.. that's not even a GP!!
 

lourai*87

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I had issues with that to start. All good in the end though. Youll find there was something similar Q4b "a function f(x) is defined by f(x) = (x + 3)(x^2 - 9)". So that let me go back and double check.

How did we work out the focus? I know a = 2 and focus = (0,a) but the (y - 1) changes that. I think i wrote (0,1)
 

lovetheoc

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lol

I was so happy to get that question, lol i probably did it wrong because i was laughing when I did it! My twim brother and I were quizzing each other on formulae and i asked him that and he was like no way that's like year 8 stuff he was so mad it was in the test
 

o0_jolie_0o

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umz if vertex is (0,1) and a = 2 then focus would be (0,3) ... i don't remember the question so just judging by you guyz answer ('a' is the focal length hence add 'a' to the vertex to get focus) if i'm wrong do correct me
 

Cloak

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DementedDonkey said:
the question was x^3-27
Answer: (x-3)(x^2 + 3x + 9)..
I got it wrong too.. purely out of exam stress, because i know how to do it but i remember writing 6x instead of 3x.. im so upset, i cant afford to lose any stupid marks like that.. i did so crap on the other parts..
i did that PLUS with teh inequality question i had

-1<=x-1<=1

for some reason i can't add and ended up with

1<=x<=4

oh yeah and their was the area sector question instead of dividin 180 by 6 i times 180 by 0.6 and was thinkin their must be something wrong with this. Oh well i pretty much have failed if im very very lucky i will get 50 and a half %

No need to study anymore.
 

Shell

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Kyroth*** said:
That question... grrrrrrrrrrr
I know difference of two squares, easy! But cubes? I spect 5min choosing random factorisations and expanding them out, to no avail!
*screems in frustration*
Can someone, please, tell me what x3+27 is factorised?
HAHAH i did the exact same thing as you!! i cantr factorise for shit. and i spent AGES coming up with stupid trial and error numbers, then expanding it out to see if it equalled the question. i never ended up getting it. :(
 

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