I spent (wasted?) several hours working this problem out years ago but saw it posted recently on Google Plus. I soon realized that either my solution predated my blog or I never actually posted my solution.

Here’s the problem and my solution is below: (ref: http://thinkzone.wlonk.com/MathFun/Triangle.htm)

Using only elementary geometry, determine angle x. Provide a step-by-step proof.

You may use only elementary geometry, such as the fact that the angles of a triangle add up to 180 degrees and the basic congruent triangle rules (side-angle-side, etc.). You may not use trigonometry, such as sines and cosines, the law of sines, the law of cosines, etc.

Spoiler Alert: Don’t read any further if you want to give this a shot yourself!

## MY SOLUTION:

1. since it corresponds to (intersection of two parallel lines by another)

2. since it corrresponds to

3. This implies by sum of triangle angles that , and hence is equilateral, giving us

4. Also notice that since it corresponds to , which now means that .

5. Now, consider that is *similar* to since they share the same angle measures.

6. Also notice that is *congruent* to since , , and they share the side *AC*. (SAS)

7. Now since then , since Corresponding Parts of Congruent Triangles are Congruent (CPCTC).

8. Now (7) together with (5), means that is *congruent *to .

9. This means that (CPCTC).

10. By (3), this means that , so that is an isosceles triangle. So, because they must be equal and .

11. Now, using the fact that (from the fact the we know and ) we just subtract this from and we have that .

Sorry, but as a Geometry teacher, I wouldn’t give you an A on your “proof”. For one thing, your terminology is off. You use the word “corresponds” in steps 1 and 2 when it appears that you really mean “alternate interior angles”. Corresponding angles are a different animal — you used it correctly in step 4. But main my beef is that you have added points F and G without stating how you got them. It appears that your first step should have been “draw segment DF parallel to segment AB”. You can then draw segment AF, which in turn defines point G, where AF and BD intersect. And you can certainly draw a segment from point A that makes a 60 degree angle with segment AB, but you haven’t shown that segment would necessarily intersect segment BC at your previous point F. Similarly, you can draw the segment that bisects angle C, but you haven’t shown that it necessarily intersects segment BD at point G. I am not saying that your answer is incorrect, just that you haven’t presented a sufficiently rigorous proof.

That’s awesome! I haven’t looked at this in years and your comments ares absolutely spot on. While the proof is sound it is CERTAINLY incomplete as you have indicated. I intuit a great deal too much integer setup. Expect a revision and thanks for your excellent feedback.

I got infinitely many solutions. 20 degrees works, as does 10, 15, 1, etc.