My Math Forum Completing the Square

 Algebra Pre-Algebra and Basic Algebra Math Forum

April 8th, 2017, 11:19 AM   #11
Senior Member

Joined: Jan 2015
From: London

Posts: 108
Thanks: 2

Quote:
 Originally Posted by skeeter $-x^2-6x+2$ $-(x^2+6x-2)$ $-\left[(x^2+6x+9) - 9 - 2\right]$ $-\left[(x+3)^2 - 11\right]$ $-(x+3)^2 + 11$
Oh do you not do this:
$\displaystyle -[(x+3)^2-9]-2$
and why not?

April 8th, 2017, 11:33 AM   #12
Math Team

Joined: Jul 2011
From: Texas

Posts: 2,750
Thanks: 1400

Quote:
 Originally Posted by jamesbrown Oh do you not do this: $\displaystyle -[(x+3)^2-9]-2$ and why not?
(1) because that is incorrect

$-[(x+3)^2-9]-2 = -(x+3)^2 + 9 - 2 = -(x+3)^2 + 7 \ne -(x+3)^2 + 11$

(2) because I prefer putting everything inside $-[bracket]$.

if you want to leave the +2 outside the $-[bracket]$ ...

$2 - 6x - x^2 = -x^2 - 6x + 2$

$-(x^2 + 6x) + 2$

$-(x^2 + 6x + 9 - 9) + 2$

$-\left[(x^2 + 6x + 9) - 9\right] + 2$

$-\left[(x+3)^2 - 9\right] + 2$

$-(x+3)^2 + 9 + 2$

$-(x+3)^2 + 11$

April 8th, 2017, 12:28 PM   #13
Senior Member

Joined: Jan 2015
From: London

Posts: 108
Thanks: 2

Quote:
 Originally Posted by skeeter (1) because that is incorrect $-[(x+3)^2-9]-2 = -(x+3)^2 + 9 - 2 = -(x+3)^2 + 7 \ne -(x+3)^2 + 11$ (2) because I prefer putting everything inside $-[bracket]$. if you want to leave the +2 outside the $-[bracket]$ ... $2 - 6x - x^2 = -x^2 - 6x + 2$ $-(x^2 + 6x) + 2$ $-(x^2 + 6x + 9 - 9) + 2$ $-\left[(x^2 + 6x + 9) - 9\right] + 2$ $-\left[(x+3)^2 - 9\right] + 2$ $-(x+3)^2 + 9 + 2$ $-(x+3)^2 + 11$
That makes a lot more sense. What I was doing, was putting inside the brackets and then out of the brackets again. Without changing the sign.

Thanks for all the help

 Tags completing, square

 Thread Tools Display Modes Linear Mode

 Similar Threads Thread Thread Starter Forum Replies Last Post Trainpancake Algebra 3 October 2nd, 2013 02:11 PM milly2012 Algebra 2 April 28th, 2012 08:44 AM bearsfan092 Algebra 3 January 24th, 2010 05:09 PM PistolPete Algebra 15 May 20th, 2009 06:44 PM SteveThePirate Algebra 4 May 17th, 2009 12:44 AM

 Contact - Home - Forums - Cryptocurrency Forum - Top