# Finding Unknown Coefficients of X

• Sep 22nd 2010, 05:16 PM
BIOS
Finding Unknown Coefficients of X
I'm trying to find Unknown Coefficients A and B in the following Identity:

$\displaystyle (2x+a)^2 = 4x^2+12x+b$

So i get:

$\displaystyle 4x^2+4ax+a^2 = 4x^2+12x+b$

$\displaystyle 4a = 12$

$\displaystyle a = 3$

$\displaystyle a^2 = b$

$\displaystyle 3^2 = b$

$\displaystyle b = 9$

The book says the answers are:

a = 3 b = 7

Am i wrong or is the book wrong? :?
• Sep 22nd 2010, 05:31 PM
Ackbeet
If that truly is the correct problem statement, then your answer is correct, and the book is wrong.
• Sep 22nd 2010, 05:40 PM
BIOS
Thanks for the reply again ackbeet! Yep that's what the book says. It's pretty late here so i wanted to make sure i wasn't just tired and missing something :P
• Sep 22nd 2010, 06:16 PM
Ackbeet
Textbooks can definitely be wrong. Some books acquire a reputation for how many mistakes they have. I have a friend who edits textbooks for a living, and he found a proof-by-example actually being promoted as a valid proof technique. Naturally, you can't expect every book to be perfect. But these days, I would expect a decent author to publish known errata somewhere. Authors, if they're generally competent, would want to know about an error. So you might pass that one along to him, if you can get a hold of him.

You're welcome. Have a good one!