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# Maths homework help! C1 complete the square watch

1. Complete the square for the expressions:

3x2-x

I tried doing this but got stuck a bit.

3(x2-1/2)2-(1/2)2

based on (x2+b/2)2-(b/2)2 the x2=3 and bx=-1 right? or am I wrong here?

PS: missed the first week cause i switched schools and I'm behind a week's worth of lessons

2. (Original post by KK.Violinist)
Complete the square for the expressions:

3x2-x

I tried doing this but got stuck a bit.

3(x2-1/2)2-(1/2)2

based on (x2+b/2)2-(b/2)2 the x2=3 and bx=-1 right? or am I wrong here?

PS: missed the first week cause i switched schools and I'm behind a week's worth of lessons

When you take the 3 out of the bracket, you've divided x^2 by 3 but not the -x. Try taking the 3 out of the bracket before you try completing the square and see how you do.
3. So would it be like this?

3(x2-1/3x)-(1/3)2

if it is that, then what about the equation since it has to be 2 for the denominator? Sorry, this is the part where I'm confused.
4. (Original post by KK.Violinist)
So would it be like this?

3(x2-1/3x)-(1/3)2

if it is that, then what about the equation since it has to be 2 for the denominator? Sorry, this is the part where I'm confused.
Hi,

Take it step by step.

3x^2 - x can be written as 3(x^2 - x/3).

Next complete the square for the bit in brackets, not forgetting to multiply it all by 3.
5. ok so;

3(x-x/3/2)2

and this makes it 3(x-1/6)2?
6. (Original post by KK.Violinist)
So would it be like this?

3(x2-1/3x)-(1/3)2

if it is that, then what about the equation since it has to be 2 for the denominator? Sorry, this is the part where I'm confused.
It may be easier to think of it like this:
Let me give you another example. Suppose you have

x^2 - 5x.

To complete the square would mean doing:
(x-(5/2))^2 - (5/2)^2. This is because when we expand the first bracket we end up with x^2 - 5x + (5/2)^2 and to get rid of the last term we minus (5/2)^2.

If instead we have something like:

6x^2 - 2x - 5, the steps would look like this:

6x^2 - 2x - 5
= 6(x^2 - x/3 - 5/6)
= 6((x - 1/6)^2 - (1/6)^2 - 5/6)) (NOTE 1/3 divided by 2 is 1/6).

In the last line make sure you multiply it all by 6. Also note that to make it easier first minus out the (1/6)^2 term (which leaves us with x^2 - x/3) and then minus 5/6 (to get x^2 - x/3 - 5/6, which when multiplied by 6 gives the original equation).

Hope this helps!

You can ask if something doesn't make sense!
7. I think I've cracked it haha.

3x^2-x

3(x^2-(x/3)/2)

3(x-1/6)^2-(1/6)^2 since x=1

3(x-1/6)^2-1/36
times by 3
3(x-1/6)^2-1/12
8. (Original post by KK.Violinist)
I think I've cracked it haha.

3x^2-x

3(x^2-(x/3)/2)

3(x-1/6)^2-(1/6)^2 since x=1

3(x-1/6)^2-1/36
times by 3
3(x-1/6)^2-1/12
I can see what you are doing but some of your working is confusing and each step needs to be equal to the previous step - see how the other poster set it out.

You can always check these by expanding again.

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Updated: September 16, 2015
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