6. If the lengths of all edges of the regular quadrilateral pyramid $P-ABCD$ are equal, and $M$ is the midpoint of edge $AB$, then the cosine value of the angle formed by the skew lines $BP$ and $CM$ is $\qquad$
Given that the midpoint of $P A$ is $N$, connect $C N, M N$, then $\angle C M N$ is the angle formed by the skew lines $B P$ and $C M$. From $\triangle A B C \cong \triangle A P C \Rightarrow C N=C M=\frac{\sqrt{5}}{2} a$, $M N=\frac{1}{2} a \Rightarrow \cos \angle C M N=\frac{\frac{1}{4}}{\frac{\sqrt{5}}{2}}=\frac{\sqrt{5}}{10}$.