# Maths QuestionWatch

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#1
Factorise 9 - 3x - 2x^2 completely.

Should be easy, but I kept ketting stupid things that didn't multiply out to make that. Like (4x - root63 + 3)(4x + root 63 + 3).

Maybe ompleting the square isn't the right way to go, but this doesn't make nice rational numbers.
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15 years ago
#2
Take the negative out to start off with to make it simpler.
2x^2 + 3x - 9 = 0
(2x - 3)(x + 3) = 0
then just put it back in.
(3 - 2x) (x + 3)
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#3
Lol of course. I didn't think to try to factorise after I'd taken the negative out!

Cheers.
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15 years ago
#4
(Original post by Coran)
then just put it back in.
(3 - 2x) (x + 3)
Why bother putting the negative back in? It gives the same roots anyway
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#5
You might want to draw the curve from its factorised form or something maybe.
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15 years ago
#6
(Original post by Pixelfairy #1)
Why bother putting the negative back in? It gives the same roots anyway
The question posed said factorise so negative needs to be put in to have sussefully factorised the given expression,
though you are right the roots are the same.
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15 years ago
#7
is that really a-level work?

no offence, but it seems pretty easy and its the same as grade b/a gcse work
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15 years ago
#8
(Original post by MC REN)
is that really a-level work?

no offence, but it seems pretty easy and its the same as grade b/a gcse work
early a-level work, yes.
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#9
Yeah I know it's very simple, I've been doing maths all day so my brain has had enough, I think by missing obvious answers it's telling me to stop revising.
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15 years ago
#10
(Original post by mik1a)
Yeah I know it's very simple, I've been doing maths all day so my brain has had enough, I think by missing obvious answers it's telling me to stop revising.
or telling you to revise more......!
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15 years ago
#11
(Original post by Katie Heskins)
or telling you to revise more......!
You'd be suprised what hours of maths can do to the brain!
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15 years ago
#12
(Original post by bono)
You'd be suprised what hours of maths can do to the brain!
I find the more maths I do, the more I get into it
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15 years ago
#13
(Original post by Katie Heskins)
I find the more maths I do, the more I get into it
Yes i definitely agree - I meant a really long revision session - i.e.) after 4 hours of maths or something.
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15 years ago
#14
(Original post by bono)
Yes i definitely agree - I meant a really long revision session - i.e.) after 4 hours of maths or something.
haha fridays=5 hours of maths for me. mmm...I remember now. That was quite mindnumbing.
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15 years ago
#15
(Original post by mik1a)
Factorise 9 - 3x - 2x^2 completely.

Should be easy, but I kept ketting stupid things that didn't multiply out to make that. Like (4x - root63 + 3)(4x + root 63 + 3).

Maybe ompleting the square isn't the right way to go, but this doesn't make nice rational numbers.
-2x^2 - 3x + 9

Answer: (2x - 3)(-x - 3) OR (3 - 2x)(x + 3)

To check; either expand to give:

-2x^2 + 3x - 6x + 9
----> -2x^2 - 3x + 9

Which was the original expression you started off with.
0
15 years ago
#16
(Original post by mik1a)
Lol of course. I didn't think to try to factorise after I'd taken the negative out!

Cheers.
Why? Just factorise with the negative, its the same thing really.
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15 years ago
#17
(Original post by Phil_C)
The question posed said factorise so negative needs to be put in to have sussefully factorised the given expression,
though you are right the roots are the same.
Why? Either will expand to give the same expression, so you have factorised that expression, whichever you use.
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15 years ago
#18
(Original post by kikzen)
early a-level work, yes.
Factorising a quadratic is A-Level work?

Year 10 work.
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15 years ago
#19
(Original post by bono)
Why? Either will expand to give the same expression, so you have factorised that expression, whichever you use.
No they dont.

(2x-3)(x+3) expands to 2x^2 +3x -9 not given in the question,
whilst
(3-2x)(x+3) expands to 9 -3x -2x^2 the expression given.

So the first is wrong in terms of expansion but both do give the same roots, namely -3 and +1.5

Having taken a factor of minus one out of teh original expression, then the correct expression to factorise is:
-{2x^2 +3x -9}
so the answers can be either:
-(2x-3)(x+3) or (3-2x)(x+3) or (2x-3)(-3-x) I would quote the second of the three, it is a neater solution.
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15 years ago
#20
I just put the negative back in because little context was given, it might of been needed to put = y to drawn a graph which would've been upside down if the negatives weren't put back in. There's a lot of transformation work in P1.
0
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