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 January 19th, 2012, 10:36 PM #1 Newbie   Joined: Jun 2011 Posts: 8 Thanks: 0 What am I doing wrong in this basic equation? Or am I wrong? Here's a function: f(x) = 3x^2 - 2x - 9 I'm asked to determine the formula if x = (x - 5) So I go ahead 3(x - 5)^2 - 2(x - 5) - 9 3(x^2 + 25) - 2x + 10 - 9 3x^2 + 75 - 2x + 10 - 9 3x^2 - 2x + 75 + 10 - 9 Final Answer: 3x^2 - 2x + 76 Except in the back of the book it says my answer should be this: 3x^2 - 32x + 76 Where the heck did that 32x come from? It seems impossible, but I know my answer has to be wrong because if I assign a value for the (x - 5), the one with a 32x appears to be the correct equation.
 January 19th, 2012, 10:43 PM #2 Senior Member     Joined: Jul 2010 From: St. Augustine, FL., U.S.A.'s oldest city Posts: 12,155 Thanks: 464 Math Focus: Calculus/ODEs Re: What am I doing wrong in this basic equation? Or am I wr Where you went wrong is saying: $(x-5)^2=x^2+25$ when actually $(x-5)^2=x^2-10x+25$ (recall $(x+y)^2=x^2+2xy+y^2$) Try it again, and you will get the result your book has.
 January 19th, 2012, 10:58 PM #3 Newbie   Joined: Jun 2011 Posts: 8 Thanks: 0 Re: What am I doing wrong in this basic equation? Or am I wr Blah, thanks, I don't know why I missed that, we just did factoring of polynomials, should have gotten that lol.

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