The Student Room Group
Complete the square, if you still don't get it, I'll give you a clue later.
Reply 2
complete the square.

take the form x^2+ax+b

complete the square will become (x+.5a)^2-(.5a)^+b
Here's an example: x^2 + 8x + 13 becomes (x+4)^2 - 16 + 13 = (x+4)^2 - 3, can you do it now? :smile:
Reply 4
You need to complete the square.
Reply 5
srose
Hi
How do I do this question:
write f(x) = x^2+6x+11 in the form (x+a)^2+b

Thanks

By completing the square...
So you go (x+3)^2-9+11
then (x+3)^2+2 ---------> so a=3 and b=2

so the answer is x= -3 and 2.
Use the general formula:

x2+ax+b(x+a2)2a24+bx^2 + ax + b \equiv (x+\frac{a}{2})^2 - \frac{a^2}{4} + b
Reply 7
Thanks for the replies.

I understand everything apart from where the -9 comes from in:

(x+3)^2-9+11
Reply 8
f(x)=x2+6x+11f(x) = x^2+6x+11

Halve the 6x6x to get 3 and shove it in the brackets (x+3)(x+3)

Now SQUARE 3 and put it outside the brackets. It must always be minus.

Therefore (x+3)29(x+3)^2 -9 and don't forget the rest of the equation:

(x+3)29+11(x+3)^2 -9 +11 becomes (x+3)2+2(x+3)^2 +2

You need to use a different method when the coefficient of x2>1x^2 > 1

Follow the example of the formula given a few posts above and you should be fine.
Reply 9
Ok, Thanks I get it now
Reply 10
Mathematician!
Use the general formula:

x2+ax+b(x+a2)2a24+bx^2 + ax + b \equiv (x+\frac{a}{2})^2 - \frac{a^2}{4} + b


a24\frac{a^2}{4}

I thought it was just a2a^2?
sam161
a24\frac{a^2}{4}

I thought it was just a2a^2?


It may not always be a whole number.

E.g

x23x+4x^2 -3x +4
(x32)2(32)2+4(x-\frac{3}{2})^2 - (\frac{3}{2})^2 +4
(x32)294+4(x-\frac{3}{2})^2 - \frac{9}{4} + 4
Look up completing the square
Reply 13
how do u square it?
where did u get 3 and the nine from. they werent even in the ques!
and if it was x^2 - 8x + 21, what wud the answer be?
Reply 14
Original post by heuygff
how do u square it?
where did u get 3 and the nine from. they werent even in the ques!
and if it was x^2 - 8x + 21, what wud the answer be?

Bumping a thread that is over 4 years old :eek:
Thanks very much!
Like thats gonna help no offence but this is a topic im personally struggling on and id prefer if u went itno more depth... btw sorry im not trying to be rude ,maths isnt my strong subject
Original post by ............d
Like thats gonna help no offence but this is a topic im personally struggling on and id prefer if u went itno more depth... btw sorry im not trying to be rude ,maths isnt my strong subject


Dude this is an 8-year-old thread. Make your own thread next time.

What bit of completing the square do you struggle with?
Reply 18
Closing the thread to prevent it being bumped again.

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