1. ## Euclidean Geometry

The attachment consists of 2 exercises from my last week's homework that I attempted to prove using numerous strategies, but never found a good one.

I searched for a while for the characteristics of a cyclic quadrilateral to use for the set up of the first proof, but never got even that far, and thus didn't get very far at all (I know...No S*^%!). As for the second exercise, no axiom, theorem, definition, or combination seemed very strong to prove the part i proposition. That left part iii pretty hopeless.

Thanks!

2. According to PlanetMath, a necessary and sufficient condition for a quadrilateral to be cyclic, is that the sum of a pair of opposite angles be equal to 180 degrees.

Concerning part 1, let F be a point on m left of C. I believe there is a theorem that angle FCA = angle ABC (if you need help finding it, let us know). We have angle ADE = 180 - CDE = 180 - FCA. Thus, ADE + ABE = 180. Similarly, it's easy to show that DAB + DEB = 180.

3. ## Curious about the theorem

Thank you very much for the help emakarov! Please forgive me if I'm being dense, but the claim that angle FCA = angle ABC is difficult for me to accept given that F is an arbitrary point on m, only constrained such that it lies to the left of C. Because angle ABC is fixed once constructed, my wee brain cannot find a satisfactory way to believe the proposition.

I searched a bit for the theorem you mentioned, but didn't get far because I really don't know what I might search for, i.e. what to guess that the theorem might be called.

Notwithstanding all that, thanks for your help.

4. Put $\displaystyle F$ on $\displaystyle m$ so that $\displaystyle A~\&~F$ are on the same side of $\displaystyle \overleftrightarrow {BC}$

Now angles $\displaystyle \angle ABC\;\& \,\angle FCA$ are congruent, they intercept the same arc.

Angles $\displaystyle \angle CDE\;\& \,\angle FCA$ are alternant interiors so are congruent.

Angles $\displaystyle \angle CDE\;\& \,\angle EDA$ are supplementary.

Therefore angles $\displaystyle \angle ABE\;\& \,\angle EDA$ are supplementary.

That proves the quadrilateral is cyclic.

5. the claim that angle FCA = angle ABC is difficult for me to accept given that F is an arbitrary point on m
Since F is on m and left of C, moving it does not change the angle FCA!

As Plato said, $\displaystyle \angle ABC$ and $\displaystyle \angle FCA$ are equal because they are half the arc AC. I found this explanation on cut-the-knot.org. You can also search for "inscribed angle".

6. Thanks! Your responses led me to finally getting the idea through my rather thick skull.

I'm about to post another question that I find really interesting. After a good bit of consideration, I haven't come up with a solution for. Hope you check it out.

Thanks again!