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# Quadratic functions Edexcel C1 watch

1. I'm new here so please go easy on me

I'm trying to solve quadratic equations with the formula ax2 + bx + c = 0

Factorising x2 - 4x - 12 = 0 is fine as the solution (x -6) (x +2) = 0 creates b by adding up -6 and +2 and c by multiplying them.

However for factorising an equation like 6x2 - 7x 3 = 0, the text book gives the solution as (3x+1) (2x-3). What happened to b (7x), you can't get it by adding up 1 and -3.

Same here: 4x2 - 16x + 15 = 0 is factorised as (2x -3) (2x -5). I would factorise it as (2x -15) (2x -1).

What am I doing wrong?

ps All examples are from Edexcel C1 2B
2. (Original post by dear.hanna)
I'm new here so please go easy on me

I'm trying to solve quadratic equations with the formula ax2 + bx + c = 0

Factorising x2 - 4x - 12 = 0 is fine as the solution (x -6) (x +2) = 0 creates b by adding up -6 and +2 and c by multiplying them.

However for factorising an equation like 6x2 - 7x 3 = 0, the text book gives the solution as (3x+1) (2x-3). What happened to b (7x), you can't get it by adding up 1 and -3.

Same here: 4x2 - 16x + 15 = 0 is factorised as (2x -3) (2x -5). I would factorise it as (2x -15) (2x -1).

What am I doing wrong?

ps All examples are from Edexcel C1 2B
You need to split the -7. To do this you need two numbers that multiply to give (6)(-3)=-18 the numbers that do this are -9 and 2
3. Hi Steve,

I can see what you did there. However what I don't understand is why Edexcel factorises the same equation as (3x +1) (2x -3). Does that mean that there's mistake in their text book? Their solution to the other one 4x2 -16x + 15 (above) doesn't really make any sense either.

I wonder if there are any more inconsistencies in the Edexcel books? Little bit worrying for me as I would obviously like to think that what's in the book is correct
4. Oh I think I can see it now. Thanks Steve.
5. (Original post by dear.hanna)
I'm new here so please go easy on me

I'm trying to solve quadratic equations with the formula ax2 + bx + c = 0

Factorising x2 - 4x - 12 = 0 is fine as the solution (x -6) (x +2) = 0 creates b by adding up -6 and +2 and c by multiplying them.

However for factorising an equation like 6x2 - 7x 3 = 0, the text book gives the solution as (3x+1) (2x-3). What happened to b (7x), you can't get it by adding up 1 and -3.

Same here: 4x2 - 16x + 15 = 0 is factorised as (2x -3) (2x -5). I would factorise it as (2x -15) (2x -1).

What am I doing wrong?

ps All examples are from Edexcel C1 2B
then you'd get 4x^2 -32x + 15

make sure you check your factorisations
6. (Original post by dear.hanna)
Same here: 4x2 - 16x + 15 = 0 is factorised as (2x -3) (2x -5).

What am I doing wrong?
This is right. Remember that

So:

Then add/subtract the coefficients of :

Hope this helps.

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