# Finding an angle within an 80-80-20 isosceles triangle

The following is a geometry puzzle from a math school book. Even though it has been a long time since I finished school, I remember this puzzle quite well, and I don't have a nice solution to it.

So here is the puzzle:

The triangle $ABC$ is known to be isosceles, that is, $AC=BC$. The labelled angles are known to be $\alpha=\gamma=20°$, $\beta=30°$. The task is to find the angle labelled "?".

The only solution that I know of is to use the sine formula and cosine formula several times. From this one can obtain a numerical solution. Moreover this number can be algebraically shown to be correct (all sines and cosines are contained in the real subfield of the 36th cyclotomic field). So in this sense I solved the problem, but the solution is kind of a brute force attack (for example, some of the polynomials that show up in the computation have coefficients > 1000000). Since the puzzle originates from a book that deals only with elemetary geometry (and not even trigonometry if I remember correctly) there has to be a more elegant solution.

• One geometric trick is add a few lines to create an equilateral triangle; then it's a straightforward geometric argument. – Robin Chapman Oct 16 '10 at 13:24
• @Robin: entering a return when commenting posts your comment. Just let it word wrap. I have made the same mistake. – Ross Millikan Oct 16 '10 at 14:58
• @Ross (and @Robin): Please ask the Stack Exchange people to fix this "feature": Pressing Enter in comment box unexpectedly submits form – ShreevatsaR Oct 16 '10 at 15:38
• I wonder if there is a way to solve it for any alpha beta and gamma. – Chao Xu Oct 16 '10 at 16:07
• I have changed the title. In future, I suggest you try to use a more appropriate title. Titles like "A geometry puzzle" are too generic. – Aryabhata Oct 16 '10 at 17:47

The solutions are not as trivial as one would expect from the statement. It's called Langley's problem.

Check this out.

• the berkeley link is kind of confusing... but +1 for the name as a key to searching! – Jason S Oct 16 '10 at 14:35

Here is the solution I had for this (I had the writeup lying around in an old email I had sent regarding this, so, no tex, also, A and B are interchanged):

We can see that an 80-80-20 triangle is nothing but a part of the triangulation of an 18 sided regular polygon, whose 6 triangles are shown in the the bottom part of the circle above.

Now consider an 80-80-20 triangle (ABC on top part of figure) and shoot a light ray from one of the base vertices (B in the figure) at an angle of 50 degrees to the base (or 30 from one of the equal sides). (See the triangle in the top part of the circle and the red arrows)

We can show that the ray will reflect twice (first at D then E) and strike at a 90 degree angle the third time (F) i.e. after 5 refections, the light ray will return back to the vertex!

This reflection process can be pictured in another way, by reflecting the triangle each time instead of reflecting the ray (see the red arrows in the 6 triangles in the bottom part)

Now the point of the third reflection F (i.e. the 90 degree incidence point) is exactly the midpoint of the side on which the ray is incident on. This can be seen by considering the bottom part:

Consider the right most B and corresponding triangle CBF. This is a 90-60-30 triangle. Thus CF is half CB which is half CA.

(Back to triangle at top) i.e. F is midpoint of AC. Thus triangle ACE is isosceles, thus angle CAE = 20.

Thus, we see that angle BDE must be the angle x in the problem, which must be 180-(50+50) = 80. (as DE is BD reflected on AC). That angle y is 30, follows…

Note: To get more context about what x and y are, this was the figure when the problem was shown to me:

I have written four distinct solutions to this problem: $$Solution Number 1:$$ $$Solution Number 2:$$ $$Solution Number 3:$$ $$Solution Number 4:$$

• I have a question in solution 1 : How do you know D' E AND C are collinear – sidt36 Feb 20 '17 at 10:50
• @sidt36, I have explained about it just under the first question. – Seyed Feb 20 '17 at 12:44
• For solution 1 Will the same procedure work if make the angles and x,80-x and y,80-y – sidt36 Feb 20 '17 at 15:04
• Solution 1: I see that CN meets AF at right angles but don't see any bisector. How come $\angle D'FA=\angle FAD'$? And, as Blue commented to mina_world, please enter text as text (using MathJax for the maths); don't just display your text in your pictures. – Rosie F Feb 6 at 12:21
• Solution 2: If you want to define $O$ by $\angle CBO=20^\circ$, fine, but you'd better phrase it as $\angle CBO=\angle BAC$. Alternatively, say "let $O$ be on $CD$ where $OB=BC$". Then the solution is J.W. Mercer's. – Rosie F Feb 6 at 12:25

This is the easiest solution. Don’t search for another.

• Let $D'$ lie on $AC$ so that $ED' \parallel BC$.
• Let $BD'$ meet $CE$ at $P$.
• As $\triangle BCP$ is equilateral, we have $CB \equiv CP$.
• As $\triangle BCD$ is isosceles, we have $CB \equiv CD$.
• Hence, $\triangle DCP$ is isosceles, which yields $\angle CPD = 80^{\circ}$ and $\angle DPD' = 40^{\circ}$.
• As $\angle DD'P = 40^{\circ}$ also, we find that $\triangle D'DP$ is isosceles, so $DD' \equiv DP$.
• As $\triangle D'EP$ is equilateral, we have $ED' \equiv EP$.
• Hence, by $\mathsf{SSS}$, we obtain $\triangle DED' \equiv \triangle DEP$.
• Therefore, $DE$ bisects $\angle D'EP$, so $\angle CED = \angle PED = 30^{\circ}$. $\quad \blacksquare$

• Math.SE has accumulated a number of duplicate $80^\circ$-$80^\circ$-$20^\circ$ questions over the years; it's refreshing to see someone post an answer. :) Please, though, enter the text of your solution as text (formatted using MathJax). Text in images is problematic; it isn't searchable, and it's useless to people who use screen readers. – Blue Jan 23 at 4:39