5. (5 points) By considering following quadratic function in $\lambda$ : $$ QC\lambda)=\sum_{i=1}^{n}\left(\lambda\left(x_{i}- \bar{x} ight)+\left(y_{i}-\bar{y} ight) ight)^{2} $$ show that the sample correlation coefficient $r$ satisfies $-1 \leq r \leq 1$. S.P.PB. 350