# Completing the square in quadratics of the form x^2 + bx + c

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• August 9th 2011, 11:16 AM
David Green
Re: Completing the square in quadratics of the form x^2 + bx + c
Quote:

Originally Posted by Siron
Can you be more clear about your question? We've found the complete square of $x^2-8x-5$ which is $(x-4)^2-21$.
Where are you stuck? What do you not understand? ...

I thought the completed square was the first part that was worked out.

x^2 - 8x - 5

(x - 4)^2 - 16 without the - 5 being added, why am I wrong?
• August 9th 2011, 11:24 AM
Siron
Re: Completing the square in quadratics of the form x^2 + bx + c
Completing the square is a technique for converting a quadratic polynomial of the form $ax^2+bx+c$ into the form $a(x-p)^2+q$ where $p,q$ are constants.
So that's what we've done here and notice $(x-4)^2-16=x^2-8x+16-16=x^2-8x$.