# Proving

• Mar 10th 2013, 03:11 AM
RuyHayabusa
Proving
How do you prove that x^2+y^2-2xy+x-y+1 is greater than 0?
• Mar 10th 2013, 03:26 AM
Prove It
Re: Proving
You can't.

\displaystyle \begin{align*} x^2 - 2\,x\,y + y^2 + x - y + 1 &= \left( x - y \right)^2 + x - y + 1 \end{align*}

All that you can say for sure is that \displaystyle \begin{align*} \left( x - y \right)^2 \end{align*} is nonnegative. The entire quantity will only be positive if \displaystyle \begin{align*} x - y + 1 > 0 \end{align*}, which you are not guaranteed.
• Mar 10th 2013, 07:25 AM
Nehushtan
Re: Proving
Quote:

Originally Posted by Prove It
You can't.

\displaystyle \begin{align*} x^2 - 2\,x\,y + y^2 + x - y + 1 &= \left( x - y \right)^2 + x - y + 1 \end{align*}

All that you can say for sure is that \displaystyle \begin{align*} \left( x - y \right)^2 \end{align*} is nonnegative. The entire quantity will only be positive if \displaystyle \begin{align*} x - y + 1 > 0 \end{align*}, which you are not guaranteed.

It does not matter if $x-y+1$ is negative; $(x-y)^2$ may be big enough to make the whole expression positive.

Indeed, $x^2+y^2-2xy+x-y+1=(x-y)^2+(x-y)+1=\left(x-y+\frac12\right)^2+\frac34$ is clearly always positive.
• Mar 10th 2013, 10:41 AM
MINOANMAN
Re: Proving
Excellent proof ...My congratulations to Nehushtan

Minoas