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August 22nd, 2012, 01:06 PM   #1
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Solve quadratic equation knowing y

Hello everybody,

Ive 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
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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...
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August 22nd, 2012, 10:21 PM   #3
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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.
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August 23rd, 2012, 12:17 AM   #4
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Re: Solve quadratic equation knowing y

You may use the quadratic formula. Suppose you have:



Write it in standard form:



Thus:

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August 23rd, 2012, 12:26 AM   #5
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Re: Solve quadratic equation knowing y

Now it was clear, thanks a lot!!!!
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August 23rd, 2012, 12:37 AM   #6
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Re: Solve quadratic equation knowing y

Whoops....didn't notice the x^2...
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August 23rd, 2012, 12:40 AM   #7
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Re: Solve quadratic equation knowing y

What do you mean now?!?!?
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August 23rd, 2012, 12:47 AM   #8
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Re: Solve quadratic equation knowing y

I think Denis is just saying he initially didn't realize the equation was quadratic.
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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?
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