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Funky function
TheUltimate123   24
N an hour ago by ezpotd
Source: CJMO 2022/5 (https://aops.com/community/c594864h2791269p24548889)
Find all functions \(f:\mathbb R\to\mathbb R\) such that for all real numbers \(x\) and \(y\), \[f(f(xy)+y)=(x+1)f(y).\]
Proposed by novus677
24 replies
TheUltimate123
Mar 20, 2022
ezpotd
an hour ago
Rolling Cube Combinatorics
KSH31415   0
Apr 24, 2025
Suppose $1\times1\times1$ die is laid down on an infinite grid of $1\times1$ squares. The die starts perfectly aligned in one of the square of the grid with the $6$ facing up and the $5$ facing forward. The die can be "rolled" along an edge such that is moves exactly one unit up, down, left, or right and rotates $90^\circ$ correspondingly in that axis.

Determine, with proof, all possible squares relative to the starting square for which it is possible to roll the die to that square and have it in the same orientation ($6$ facing up and $5$ facing forward).

(This problem may be hard to visualize/understand. If you don't understand the question, please ask and I can clarify.)
0 replies
KSH31415
Apr 24, 2025
0 replies
PIE Help
Rice_Farmer   5
N Mar 19, 2025 by Rice_Farmer
How many ways are there to color the $8$ regions of a three-set Venn Diagram with $3$ colors such that each color is used at least once? Two colorings are considered the same if one can be reached from the other by rotation and reflection.

Non ai way btw pls
5 replies
Rice_Farmer
Mar 15, 2025
Rice_Farmer
Mar 19, 2025
Simple Combinatorics on seating in a circle
duttaditya18   6
N Mar 6, 2025 by S_14159
Five persons wearing badges with numbers $1, 2, 3, 4, 5$ are seated on $5$ chairs around a circular table. In how many ways can they be seated so that no two persons whose badges have consecutive numbers are seated next to each other? (Two arrangements obtained by rotation around the table are considered different)
6 replies
duttaditya18
Aug 11, 2019
S_14159
Mar 6, 2025
[PMO25 Qualifying I.14] Grid Division
kae_3   0
Feb 23, 2025
How many ways are there to divide a $5\times5$ square into three rectangles, all of whose sides are integers? Assume that two configurations which are obtained by either a rotation and/or a reflection are considered the same.

$\text{(a) }10\qquad\text{(b) }12\qquad\text{(c) }14\qquad\text{(d) }16$

Answer Confirmation
0 replies
kae_3
Feb 23, 2025
0 replies
extended ray to get a desired angle & ratio
SYBARUPEMULA   2
N Feb 5, 2025 by SYBARUPEMULA
Given triangle $ABC$. Points $D$ and $E$ at segment $BC$ and $AC$ respectively such that $BD = 2CD$ and $AE = CE$. The ray $ED$ is extended to point $F$ such that $\angle$$ABC = \angle$$EFC$. It's given $DE = 8DF$, compute $\frac{AC}{BC}$.
2 replies
SYBARUPEMULA
Jul 13, 2024
SYBARUPEMULA
Feb 5, 2025
Questions about angle notation
tsun26   2
N Jan 13, 2025 by AbhayAttarde01
What's the difference between writing $\angle BAL \cong \angle DAC$ and $\angle BAL = \angle DAC$?
2 replies
tsun26
Jan 13, 2025
AbhayAttarde01
Jan 13, 2025
Line of slope i
eduD_looC   1
N Dec 19, 2024 by MineCuber
What happens when you rotate a line of slope $i$ by $\theta$ degrees? All I've got is that if $\theta=90$, then the new line should still have slope $i$ since $i \times i = -1$. :/

On a second thought, is it even possible to rotate such a line?

I remember reading about this because "five points determine a conic" and "three points are sufficient to determine a circle". So there are two special points on all circles (smth like that).
1 reply
eduD_looC
Dec 19, 2024
MineCuber
Dec 19, 2024
Escape from the genie
talkinaway   8
N Oct 28, 2024 by parmenides51
A genie has tied and blindfolded you, and has placed four coins in front of you on a square plate's vertices - the coins do not start out all heads or all tails.

The genie makes a deal with you: When the coins are all heads or all tails, he will release you, but until then, you are bound and blindfolded. You may turn any number of coins you like, and say "ALAKAZAM" to indicate you are ready for the genie to inspect the coins. The genie will inspect the coins, and will either free you (if the coins are right), or will rotate the table $90, 180, 270, 360$ degrees - you won't know which.

After he rotates the table, you may flip coins again, and shout "ALAKAZAM" again. You may repeat this as many times as you wish.

You realize that there's a way to escape - and this way to escape requires you shouting "ALAKAZAM" at most $N$ times. Find the minimal value of $N$, and describe the strategy.
8 replies
talkinaway
Apr 10, 2013
parmenides51
Oct 28, 2024
Knights at the Round Table
mathkid   48
N Sep 29, 2024 by ehuseyinyigit
Twenty five of King Arthur's knights are seated at their customary round table. Three of them are chosen - all choices of three being equally likely - and are sent off to slay a troublesome dragon. Let $P$ be the probability that at least two of the three had been sitting next to each other. If $P$ is written as a fraction in lowest terms, what is the sum of the numerator and denominator?
48 replies
mathkid
Mar 13, 2004
ehuseyinyigit
Sep 29, 2024
decimal notation
FirstUndertale   1
N Aug 10, 2024 by Iwowowl253
Consider the real number, written in decimal notation,
\[ r = 0.235831\ldots \]
where, starting from the third decimal place after the comma, each digit is equal to the remainder when the sum of the two previous digits is divided by 10. We can state that:

(a) \((10^{60} - 1) \cdot r\) is an integer

(b) \((10^{25} - 1) \cdot r\) is an integer

(c) \((10^{17} - 1) \cdot r\) is an integer

(d) \(r\) is an algebraic irrational number

(e) \(r\) is a transcendental (non-algebraic) irrational number
1 reply
FirstUndertale
Aug 10, 2024
Iwowowl253
Aug 10, 2024
a