GCSE Edexel maths completing the square

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Mavs04
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Why is it not 25/16 here, isn’t 5/4 squared 25/16, not 25/Name:  538E1707-34D7-4042-AE75-7434288DDD7F.jpeg
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RDKGames
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(Original post by Mavs04)
Why is it not 25/16 here, isn’t 5/4 squared 25/16, not 25/8?
Because of the factor of 2 at the very beginning.


2(x+\frac{5}{4})^2 = 2 ( x^2 + \frac{5}{2}x + \frac{25}{16}) =  ???
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Mavs04
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This is the way I did it in decimals and the answer turned out right, I’m not sure why the fraction is like that thoughName:  image.jpg
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Mavs04
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(Original post by Mavs04)
This is the way I did it in decimals and the answer turned out right, I’m not sure why the fraction is like that thoughName:  image.jpg
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Would it not just be 2[(x +5/4)^2 - 25/16] +9 ???
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Mavs04
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(Original post by Mavs04)
Would it not just be 2[(x +5/4)^2 - 25/16] +9 ???
Wait I think I’ve got it:

= 2(x+5/4)^2-50/16 + 144/16
= 2(x+5/4)^2 + 94/16
= “””””””””””””” + 47/8


???
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simon0
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Okay, a few points:

POST 1:
As RDKGames has mentioned (in the second post of this discussion), (25/16) was multiplied by 2 to obtain (25/8):

 2 \big( x + \big( \frac{5}{4} \big) \big) ^{2} = 2 \big[ x^{2} + \frac{5}{2} x + \frac{25}{16} \big] , \\

= 2(x^{2}) + 2 \big( \big( \frac{5}{2} \big) x \big) + 2 \big( \frac{25}{16} \big) , \\

= 2x^{2} + 5x + \big( \frac{25}{8} \big).

POST 3:
Best to stick with fractional representation for this question as you may deal with non-ending decimal representations and it is clearer with fractions.

Also, on the third line of your script:

 2(x^{2} + 2.5x) \neq 2 [ (x+1.25) - 1.5625] + 9.

You have forgotten to square the (x + 1.25) term on this line and the lines afterwards.

POST 4:
Yes it is:  2[(x +(5/4))^{2} - 25/16] +9.

POST 5:
Yes but it is best to break down (50/16) to (25/8) so to be easier to malipulate the other terms.
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