5. The continuation of the bisector $C D$ of triangle $A B C$ intersects the circumcircle of this triangle at point $E$. The circumcircle of triangle $A D E$ intersects the line $A C$ at point $F$, different from $A$. Find $B C$, if $A C=b, A F=a$.
5. If $A$ is between $C$ and $F$, then $B C=a+b$, if $F$ is between $A$ and $C$, then $B C=b-a$.