pull down to refresh

The two circles O and O^\prime with radii of 1 are tangent with each other and with the horizontal line at the bottom. Circle O_1 is inscribed with a maximum possible radius inside O, O^\prime, and the horizontal line. O_2, O_3,... are successively inscribed following the same rule maximizing their radius.
Find the diameter of a circle O_n.
Previous iteration: #720410 (quite a few different valid solutions were found for this one, congrats~~)
For O_1 looking at the right triangle
Pythagoras says
(1-r)^2 + 1^2 = (1+r)^2
which means the diameter of O_1 is \frac{1}{2}.
This can be repeated of O_2 giving \frac{1}{6} and O_3 giving \frac{1}{12} etc.
I don't yet have the generic formula for O_n
reply
Looks good. I'm not sure about your value for O_3 though.
reply
For O_2
(1 - \frac{1}{2} - r)^2 + 1 = 1 + 2*r + r^2
For O_3
(1 - \frac{1}{2} - \frac{1}{6} - r)^2 + 1 = 1 + 2*r + r^2
(\frac{6}{6} - \frac{3}{6} - \frac{1}{6} - r)^2 + 1 = 1 + 2*r + r^2
(\frac{1}{3} - r)^2 + 1 = 1 + 2*r + r^2
\frac{1}{9} - \frac{2}{3}*r + r^2 + 1 = 1 + 2*r + r^2
\frac{1}{9} - \frac{2}{3}*r = 2*r
\frac{1}{9} = \frac{8}{3}*r
\frac{3}{9*8} = r
\frac{1}{24} = r
Diameter is twice that, so \frac{1}{12}
No?
reply
That would give diameters \frac{1}{2}, \frac{1}{6}, \frac{1}{12}, \frac{1}{20}, \frac{1}{30}, ...
For the sequence of divisors there are multiple candidates
reply
This is a great website. Didn't know about it. Made me check out the infamous https://oeis.org/A000127 sequence, for which there seem to be many more cases showing this unexpected change from 32 to 31.
reply
Aren't you using the same symbol r to denote the different radii of O_1 (as in the drawing) and O_2 (as in the right-hand side of the formula below)? And same comment for the subsequent O_3 equation?
For O_2
(1 - \frac{1}{2} - r)^2 + 1 = 1 + 2*r + r^2
For O_3
(1 - \frac{1}{2} - \frac{1}{6} - r)^2 + 1 = 1 + 2*r + r^2
I used a different approach to solve this problem (I'll post it as a solution with the next iteration), but that one gives me 1/18 for O_3.
reply
For O_3
(the top-left corner of the triangle is supposed to be the center of O)
The hypotenuse is 1 + r, where r denotes the radius of O_3.
The horizontal leg is simply 1 (this is true for all O_n).
The vertical leg is 1 minus the sum of all diameters O_1 and O_2 minus r of O_3.
So, Pythagoras gives r of O_3.
reply
Ok got it. This seems correct. I'll have to figure out why my approach is giving a different result. I likely must have applied it incorrectly. Please don't spend more time on this, I'm assuming the error must be on my side at this point.
I'll check my notes tomorrow.
reply