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January 8th, 2014, 09:28 AM  #1 
Newbie Joined: Jan 2014 Posts: 4 Thanks: 0  Completing the Square question.
Hi, I'm currently learning about quadratic equations and 'Completing the Square'. Basically, I need to know what I am doing it correct. I have this equation: x^2 + 5x + 2 = 0 I have worked out that (be it correct or incorrect) that: x = 3.207 or x = 1.792 But when I input one of these numbers into the equation, I don't get an answer that is zero. Have I answered this question correctly? And if so, how can I check I have? If not, what have I done wrong? I can take a photo of my workings out and post them if needs be. Many Thanks 
January 8th, 2014, 09:58 AM  #2  
Math Team Joined: Mar 2012 From: India, West Bengal Posts: 3,871 Thanks: 86 Math Focus: Number Theory  Re: Completing the Square question. Quote:
 
January 8th, 2014, 10:01 AM  #3  
Math Team Joined: Dec 2006 From: Lexington, MA Posts: 3,267 Thanks: 408  Re: Completing the Square question. Hello, GuyDoingMathFoiled! Quote:
It's hard to see your work from here, but it looks like you played the instead of the  
January 9th, 2014, 07:05 AM  #4 
Newbie Joined: Jan 2014 Posts: 4 Thanks: 0  Re: Completing the Square question.
I don't understand what I'm doing wrong here. I feel so dumb. Here is another equation I've got which for some reason I'm doing wrong. I'll type out my workings out in a hope someone will be able to point me in the right direction. (I don't know how to use the proper mathematical symbols on this forum so bear with me). x^2 3x  4 = 0 x^2 3x = 4 x^2 3x (3/2)^2 = 4 (3/2)^2 (x  3/2)^2 = 7/4 x  3/2 = ±?7/4 x = 3/2 ±?7/4 x = 0.177 or 2.823 If the above is too messy to read, please point me towards a site I can convert it into mathematic expressions. Thanks for the help! 
January 9th, 2014, 07:49 AM  #5 
Global Moderator Joined: Oct 2008 From: London, Ontario, Canada  The Forest City Posts: 7,950 Thanks: 1141 Math Focus: Elementary mathematics and beyond  Re: Completing the Square question.
In line 3, try adding (3/2)^2 instead of subtracting it.

January 9th, 2014, 08:15 AM  #6  
Newbie Joined: Jan 2014 Posts: 4 Thanks: 0  Re: Completing the Square question. Quote:
 
January 9th, 2014, 08:19 AM  #7  
Member Joined: Dec 2013 Posts: 82 Thanks: 0  Re: Completing the Square question. Quote:
You can only subtract it from the LHS. And then you can move it to the other side by adding it to both sides. Personally, I prefer doing it this way than adding it to both sides. It's a question of preference I guess. Had to edit that a couple of times to make it look correct!  
January 9th, 2014, 10:33 AM  #8  
Newbie Joined: Jan 2014 Posts: 4 Thanks: 0  Re: Completing the Square question. Quote:
Okay, but (and I'm not disputing what you say, just asking to understand better), here: You subtract from the left hand side, to make the equation equal, do you not also need to subtract from the right hand side?  
January 9th, 2014, 11:12 AM  #9 
Global Moderator Joined: Oct 2008 From: London, Ontario, Canada  The Forest City Posts: 7,950 Thanks: 1141 Math Focus: Elementary mathematics and beyond  Re: Completing the Square question. See? 
January 9th, 2014, 12:26 PM  #10  
Member Joined: Dec 2013 Posts: 82 Thanks: 0  Re: Completing the Square question. Quote:
OK, let's just look at the LHS here OK? We started with When we square it we get Let's take the first term here: and expand it. Notice that we have 9/4 too much!!! That's why we have to subtract it. Hence: That is why you don't put it on the RHS of the equation as well. The equation is still balanced. Hope that helps.  

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