Online Course Discussion Forum


Picture of Derek Dong
by Derek Dong - Saturday, September 29, 2018, 8:29 AM

How to even start example 5? I tried angle chase for two cyclic quadrilaterals but I couldn't find any angles involving only the ' points. Also, for 6 I'm trying to prove BEAF is a parallelogram but I cant find any angles with E or F.

Picture of Areteem Professor
by Areteem Professor - Saturday, September 29, 2018, 5:09 PM

For Example 5, because of symmetry, if you can prove that $G',H',I',J'$ are concyclic, then by the same reason $F',H',I',J'$ are concyclic, and then of course all five points are on the same circle.

Angle chasing for $G',H',I',J'$ involves some other cyclic quadrilaterals.  For example, try to prove that $A, B, F, G'$ are concyclic.  Then by the same reason, so are $A, B, F, J'$.  You can try to fill the details.  We will discuss more in class,

For Example 6, instead of trying to prove $AEBF$ is a parallelogram, consider using properties of midpoints.  We want to prove $O$ is the midpoint of $EF$.  Draw a perpendicular line from $O$ to $CD$ with foot $G$, then $G$ is the midpoint of $CD$.  Unfortunately points $G$ and $O$ are not in the same triangle, so we need to try to find another point to relate them...  Again, discuss more in class.