Hi guys. I'm not quite sure if this belongs in here but here goes.

We must prove that $\displaystyle y=a+bx$ for $\displaystyle b<0$ is perfectly negatively correlated.

Here is what I know. I know that a correlation coefficient $\displaystyle r(x,y) = \frac{cov Rx,Ry}{(sigmaX)(sigmaY)}$

I have the answer but i do not get where or why there is an E in there. If anyone could explain this it would be fantastic. Thank you

Chris