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# Completing the square with a negative coefficient?

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1. Completing the square:

10x - x2

My working:

10x - x2 = -x2 + 10x

= - (x2 - 10x)
= - [(x - 5)2 - 25]
= (x - 5)2 + 25

I feel like I did something wrong. Where did I go wrong?
2. (Original post by Baaah)
Completing the square:

10x - x2

My working:

10x - x2 = -x2 + 10x

= - (x2 - 10x)
= - [(x - 5)2 - 25]
= (x - 5)2 + 25

I feel like I did something wrong. Where did I go wrong?
Last step. Do it again. Where did the minus go?

Should be: -(x - 5)2 + 25

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Updated: September 18, 2016
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