Two lines $GE$ and $FD$ meet in $A$. They cut each other at $45$ degrees. Both $G$ and $F$ lie on the circumcircle of square $ALBK$ such that $E$ and $D$ lie on the $KB$ and $LB$ respectively without lying on the corners of the square. I'm supposed to prove that $GF$ and $ED$ are parallel. 1) How does one prove parallelism of lines in general? 2) $45$ degrees seems rather specific, could the same be proven with any angle? (i.e. $20$ degrees)