Number of Equilateral triangles in circle with 42 evenly spaced points? I know that the answer is 42/3 = 14 points, or in general for a circle with N points it is N/3, but I don't know why it actually works.
Why is the number of equilateral triangles for a circle with N points simply N/3? What happens when N is not divisible by 3?
 A: Measure the central angles, say, from one of the given points. We have points exactly at angles $360^\circ\cdot \displaystyle\frac kN$ with $k=0,...,N-1$.
If $N$ is not divisible by $3$, then try to prove that the difference of any two angles cannot be $120^\circ$ which would be required for the equilateral triangle.
If $3|N$, then look at the vertex which is on the first third, i.e. with angle in $[0^\circ,\,120^\circ)$. An equilateral triangle must have exactly one vertex on each third of the circle, and one vertex already determines the other two.
Now, the vertex on the first third can be any out of those $N/3$.
A: Let $x_1,\cdots,x_n$ be the points taken clockwise. If your triangle is $x_i,x_j,x_k$ then the angle at $x_k$ is $\frac{\pi}{n}|i-j|$ (it is easy to see with a picture), so it follows that $n$ is a multiple of $3$. This also shows that your points are evenly spaced (with fixed separation) which implies your claim.
This could be useful http://en.wikipedia.org/wiki/Inscribed_angle#Property.
