The Student Room Group

Factorisation question?????????????

how do you factorise : 6a^2 x-abx-12b^2 x
Reply 1
I'm not a 100% sure but this is what I would do:

You have: y = 6a^2x - abx - 12b^2x
First thing I would do is take x out:
y = x (6a^2 - ab - 12b^2)

Then you can take two approaches, either factorize it regularily, by taking (a - b) (a - b) and respectfully changing the signs and the number or you could use the formula:

(-b (plus/minus)(square root of b^2 - 4ac)) / 2a = x

In this case you have the variables: a = 6, b = -1 and c = -12, as seen in your equation above.

If you plug in the values you will get:

x = 1 (plus/minus) (root of: (-1)^2 - 4*6*(-12)) / 2*6
x = 1 (plus/minus) (root of: 1 + 288) / 12
x = 1 (plus/minus) (root of: 289) / 12

you can find the root of 289, which is 17 and put it in:

x = 1 (plus/minus) 17 / 12

From this you will get two values:

x = (1 + 17)/ 12 and x = (1 - 17)/ 12

Thus you will get two values for x, being x = 3/2 and x = -4/3

Thus;

y = x (a -3/2b) (a + 4/3b)

or, more neatly:

y = x (2a -3b) (3a + 4b)

If you want to check it would give:

y = x ((2a)(3a)+(2a)(4b)+(-3b)(3a)+(-3b)(4b))
y = x (6a^2 + 8ab - 9ab -12b^2)
y = x (6a^2 - ab -12b^2)

And

y = 6a^2x - abx -12b^2x

I hope this helped. :smile:
Reply 2
Original post by emmalav
how do you factorise : 6a^2 x-abx-12b^2 x


Take the x out and you are left with

6a2ab12b26a^2 - ab - 12b^2

Are you familiar with the method 6 times -12 = -72

Then -9 x 8 = -72
And -9 x 8 = -1

So you have

x(6a29ab+8ab12b26a^2 - 9ab + 8ab - 12b^2) = x(3a(2a3b)+4b(2a3b)3a(2a -3b) + 4b(2a - 3b)) = x(3a+4b)(2a3b)(3a+4b)(2a-3b)
(edited 11 years ago)
^^ cant just take out x, the x is still there, need to write it in
Original post by TenOfThem
Of course it is still there

I do not believe that I suggested otherwise

I showed a method not an answer


but your last line read (3a+4b)(2a-3b) ... where was the x?
obviously i can see that you knew it but dont leave it out yeh?
Reply 5
Original post by Blake-inator
...
Original post by Mr M
...


Happy now?

:smile:
Original post by TenOfThem
Happy now?

:smile:


For now but make sure you don't leave it out next time yeh?
Reply 7
Original post by Mr M
For now but make sure you don't leave it out next time yeh?


yeh

I is lurnt me lesson innit
Reply 8
Original post by TenOfThem
yeh

I is lurnt me lesson innit


leave it out guv!
Reply 9
Original post by Dornishman
I'm not a 100% sure but this is what I would do:

You have: y = 6a^2x - abx - 12b^2x
First thing I would do is take x out:
y = x (6a^2 - ab - 12b^2)

Then you can take two approaches, either factorize it regularily, by taking (a - b) (a - b) and respectfully changing the signs and the number or you could use the formula:

(-b (plus/minus)(square root of b^2 - 4ac)) / 2a = x

In this case you have the variables: a = 6, b = -1 and c = -12, as seen in your equation above.

If you plug in the values you will get:

x = 1 (plus/minus) (root of: (-1)^2 - 4*6*(-12)) / 2*6
x = 1 (plus/minus) (root of: 1 + 288) / 12
x = 1 (plus/minus) (root of: 289) / 12

you can find the root of 289, which is 17 and put it in:

x = 1 (plus/minus) 17 / 12

From this you will get two values:

x = (1 + 17)/ 12 and x = (1 - 17)/ 12

Thus you will get two values for x, being x = 3/2 and x = -4/3

Thus;

y = x (a -3/2b) (a + 4/3b)

or, more neatly:

y = x (2a -3b) (3a + 4b)

If you want to check it would give:

y = x ((2a)(3a)+(2a)(4b)+(-3b)(3a)+(-3b)(4b))
y = x (6a^2 + 8ab - 9ab -12b^2)
y = x (6a^2 - ab -12b^2)

And

y = 6a^2x - abx -12b^2x

I hope this helped. :smile:

thank you

Quick Reply

Latest