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 November 2nd, 2014, 09:53 PM #1 Newbie   Joined: Jun 2014 From: U.S.A Posts: 13 Thanks: 0 This equation has me stumped This is what I basically did: 25-x^2=(x+5)(x-5) which means multiply 6 by x-5 = 6x - 30. Multiply -9 by X+5 = -9x - 45. Combine them gets me -75-3x=8x. Add 3 gets 11x/-75. giving me a final answer of -75/11 What did i do wrong?!?! Last edited by HelpMeNow; November 2nd, 2014 at 10:14 PM. November 2nd, 2014, 10:17 PM   #2
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Quote:
 Originally Posted by HelpMeNow  This is what I basically did: 25-x^2=(x+5)(x-5) which means multiply 6 by x-5 = 6x - 30. Multiply -9 by X+5 = -9x - 45. Combine them gets me -75-3x=8x. Add 3 gets 11x/-75. What did i do wrong?!?!
$$\frac{6}{x+5}-\frac{9}{x-5}=\frac{6(x-5)-9(x+5)}{x^2-25}=\frac{-3x-75}{x^2-25}$$

So:
$$\frac{-3x-75}{x^2-25}=\frac{8x}{25-x^2}=\frac{-8x}{x^2-25}$$

So if $x^2-25 \ne 0$, you have $-3x-75=-8x$

CB November 2nd, 2014, 10:45 PM   #3
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 Originally Posted by CaptainBlack $$\frac{6}{x+5}-\frac{9}{x-5}=\frac{6(x-5)-9(x+5)}{x^2-25}=\frac{-3x-75}{x^2-25}$$ So: $$\frac{-3x-75}{x^2-25}=\frac{8x}{25-x^2}=\frac{-8x}{x^2-25}$$ So if $x^2-25 \ne 0$, you have $-3x-75=-8x$ CB
What? November 2nd, 2014, 11:13 PM   #4
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Quote:
 Originally Posted by HelpMeNow What?
What "What"????  November 3rd, 2014, 01:38 AM   #5
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Quote:
 Originally Posted by HelpMeNow What?
Your explanation of what you did is incomprehensible, as far as I can tell you lost a minus sign in the wash and you did not observe that you had to assume that $x^2\ne 5$ as otherwise both sides of the equation you are trying to solve are undefined.

CB November 3rd, 2014, 02:35 AM   #6
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Quote:
 Originally Posted by CaptainBlack you had to assume that $\displaystyle x^2\ne 5$
(Presumably, you meant $\displaystyle x^2\ne 25$) November 3rd, 2014, 04:56 AM   #7
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 Originally Posted by Hoempa (Presumably, you meant $\displaystyle x^2\ne 25$)
Yes, it was/is correct in the original response.

CB November 3rd, 2014, 05:00 AM   #8
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Quote:
 Originally Posted by HelpMeNow What?
$\displaystyle \frac{8x}{25 - x^2} = \frac{8x}{-(x^2 - 25)} = \frac{-8x}{x^2 - 25}$

This is so that the denominators on the LHS and RHS are exactly equivalent, so you can equate the numerators. Consequently, you have $\displaystyle -3x - 75 = -8x$ instead of $\displaystyle -3x-75 = 8x$.

Last edited by Benit13; November 3rd, 2014 at 05:02 AM. Tags equation, stumped Thread Tools Show Printable Version Email this Page Display Modes Linear Mode Switch to Hybrid Mode Switch to Threaded Mode Similar Threads Thread Thread Starter Forum Replies Last Post Vitals Algebra 1 August 29th, 2014 10:24 AM mmcdonald Calculus 3 March 1st, 2012 05:28 AM tangosix Algebra 2 March 2nd, 2010 10:39 AM yabo2k Advanced Statistics 1 October 27th, 2009 05:41 AM bretttolbert Algebra 4 April 5th, 2008 12:15 PM

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