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August 22nd, 2012, 01:06 PM  #1 
Newbie Joined: Aug 2012 Posts: 4 Thanks: 0  Solve quadratic equation knowing y
Hello everybody, I´ve forgot a little bit of algebra and I need some help. I made in excel a calibration curve and I got this polynomial equation: y = 4E07x2  0,0021x + 6,4089 If I know y how can I solve x? thanks 
August 22nd, 2012, 07:01 PM  #2  
Math Team Joined: Oct 2011 From: Ottawa Ontario, Canada Posts: 13,336 Thanks: 938  Re: Solve quadratic equation knowing y Quote:
x = (4E07x2 + 6,4089  y) / 0,0021 Any idea why you were stuck on that one? Quite basic...  
August 22nd, 2012, 10:21 PM  #3 
Newbie Joined: Aug 2012 Posts: 4 Thanks: 0  Re: Solve quadratic equation knowing y
Ok but in this way when we suppose that y= 5 and we substitute it, we can't also calculate x because we also have x^2, or there are other steps? Thanks a lot. 
August 23rd, 2012, 12:17 AM  #4 
Senior Member Joined: Jul 2010 From: St. Augustine, FL., U.S.A.'s oldest city Posts: 12,204 Thanks: 511 Math Focus: Calculus/ODEs  Re: Solve quadratic equation knowing y
You may use the quadratic formula. Suppose you have: Write it in standard form: Thus: 
August 23rd, 2012, 12:26 AM  #5 
Newbie Joined: Aug 2012 Posts: 4 Thanks: 0  Re: Solve quadratic equation knowing y
Now it was clear, thanks a lot!!!!

August 23rd, 2012, 12:37 AM  #6 
Math Team Joined: Oct 2011 From: Ottawa Ontario, Canada Posts: 13,336 Thanks: 938  Re: Solve quadratic equation knowing y
Whoops....didn't notice the x^2...

August 23rd, 2012, 12:40 AM  #7 
Newbie Joined: Aug 2012 Posts: 4 Thanks: 0  Re: Solve quadratic equation knowing y
What do you mean now?!?!?

August 23rd, 2012, 12:47 AM  #8 
Senior Member Joined: Jul 2010 From: St. Augustine, FL., U.S.A.'s oldest city Posts: 12,204 Thanks: 511 Math Focus: Calculus/ODEs  Re: Solve quadratic equation knowing y
I think Denis is just saying he initially didn't realize the equation was quadratic. 
August 23rd, 2012, 06:45 AM  #9  
Global Moderator Joined: Dec 2006 Posts: 19,732 Thanks: 1810  Quote:
 

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