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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 = 4E-07x2 - 0,0021x + 6,4089 If I know y how can I solve x? thanks
August 22nd, 2012, 07:01 PM   #2
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Re: Solve quadratic equation knowing y

Quote:
 Originally Posted by alexmulo y = 4E-07x2 - 0,0021x + 6,4089 If I know y how can I solve x?
0,0021x = 4E-07x2 + 6,4089 - y

x = (4E-07x2 + 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: 12,193 Thanks: 504 Math Focus: Calculus/ODEs Re: Solve quadratic equation knowing y You may use the quadratic formula. Suppose you have: $y=ax^2+bx+c$ Write it in standard form: $ax^2+bx+c-y=0$ Thus: $x=\frac{-b\pm\sqrt{b^2-4a(c-y)}}{2a}$
 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: 12,904 Thanks: 883 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: 12,193 Thanks: 504 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
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Quote:
 Originally Posted by alexmulo I made in excel a calibration curve
What were the smallest and largest values of x that you used?