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# Factorising Help Please watch

1. How do I factorise the following by completing the square ?

i ) 6x^2 - 5x - 6

ii) 14 + 45x - 14x^2

============

I know how to factorise them normally, but I don't know where to begin by completing the square. Eg Do I bring the 6 out of the first question and then complete the square ?
2. you can either divide by the coefficient of x^2, or express as (roota x +b divided by 2root a)
3. (Original post by Gelato)
How do I factorise the following by completing the square ?

i ) 6x^2 - 5x - 6

ii) 14 + 45x - 14x^2

============

I know how to factorise them normally, but I don't know where to begin by completing the square. Eg Do I bring the 6 out of the first question and then complete the square ?
For 6x^2 - 5x - 6
Yes you bring 6 out first to make 1 as cofficient of x^2
so it will be 6 [x^2-(5/6)x-1]
then do completing square...
For.equation in form of x^2 + bx + c , completing square all you do is..
[x+b/2]-(b/2)^2 + c

if b is negative then it will be [x-b/2]^2 - (b/2)^2 +c

if c is negative then you put negative before c

but you have to put the negative sign before (b/2)^2
regardless the values of b.

for second question multiply the whole equation by -1 then proceed same way.
Hope this help.
4. Thanks, I'll try do it now .

I get:

x^2 - 5/6 - 1

(x - 5/12)^2 - 25/144 - 1

(x - 5/12)^2 - 119/144

Do I just take the bracket part now and use that as the answer ?
5. shouldn't it be (x-5/12)^2 -169/144? (you subtracted 25 from 144, rather than adding). this is then an expression of the form y^2 - a^2, for constant a, which factorises as the difference of two squares. i'd imagine the other is similar, but cba to work it out!
6. (Original post by Gelato)
Thanks, I'll try do it now .

I get:

x^2 - 5/6 - 1

(x - 5/12)^2 - 25/144 - 1

(x - 5/12)^2 - 119/144

Do I just take the bracket part now and use that as the answer ?
Sorrry if this is too late reply..
But umm you should subtract 1 from -25/144 not add.
and you totalli forgot the forst '6' which u took out in beginning so it will be
6[(x-5/12)^2 -169/144]
now open bracket
6(x-5/12)^2 - 6*(169/144)
Try doing the whole again once more and you will get it correct.

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