The Difference of Two Cubes (1 Viewer)

[Kyroth***]

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?

 

[Kazness]

x^3 - 27 = (x-3)(x^2+3x+9)

 

[sikeveo]

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

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

 

[b-rex]

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

 

[DementedDonkey]

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]

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]

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]

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

 

[goan_crazy]

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]

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?

 

[Yaz88]

na i think that it was (0,3)

 

[seremify007]

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!!

 

[switchblade87]

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

Yeah me too, it sucks major horse dong.

 

[anthea1304]

ditto ditto ditto, and it was only the 2nd question! arg

 

[lourai*87]

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)

 

[seremify007]

I put (0,2) but now I realised I forgot that the vertex isn't at 0,0.

 

[lovetheoc]

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]

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]

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]

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.