# 2017 Indonesia MO Problems

## Day 1

### Problem 1 $ABCD$ is a parallelogram. $g$ is a line passing $A$. Prove that the distance from $C$ to $g$ is either the sum or the difference of the distance from $B$ to $g$, and the distance from $D$ to $g$.

### Problem 2

Five people are gathered in a meeting. Some pairs of people shakes hands. An ordered triple of people $(A,B,C)$ is a trio if one of the following is true:

• A shakes hands with B, and B shakes hands with C, or
• A doesn't shake hands with B, and B doesn't shake hands with C.

If we consider $(A,B,C)$ and $(C,B,A)$ as the same trio, find the minimum possible number of trios.

### Problem 3

A positive integer $d$ is special if every integer can be represented as $a^2 + b^2 - dc^2$ for some integers $a, b, c$.

• Find the smallest positive integer that is not special.
• Prove 2017 is special.

### Problem 4

Determine all pairs of distinct real numbers $(x, y)$ such that both of the following are true: \begin{align*} x^{100} - y^{100} &= 2^{99} (x-y) \\ x^{200} - y^{200} &= 2^{199} (x-y) \end{align*}

## Day 2

### Problem 5

A polynomial $P$ has integral coefficients, and it has at least 9 different integral roots. Let $n$ be an integer such that $|P(n)| < 2017$. Prove that $P(n) = 0$.

### Problem 6

Find the number of positive integers $n$ not greater than 2017 such that $n$ divides $20^n + 17k$ for some positive integer $k$.

### Problem 7

Let $ABCD$ be a parallelogram. $E$ and $F$ are on $BC, CD$ respectively such that the triangles $ABE$ and $BCF$ have the same area. Let $BD$ intersect $AE, AF$ at $M, N$ respectively. Prove there exists a triangle whose side lengths are $BM, MN, ND$.

### Problem 8

A field is made of $2017 \times 2017$ unit squares. Luffy has $k$ gold detectors, which he places on some of the unit squares, then he leaves the area. Sanji then chooses a $1500 \times 1500$ area, then buries a gold coin on each unit square in this area and none other. When Luffy returns, a gold detector beeps if and only if there is a gold coin buried underneath the unit square it's on. It turns out that by an appropriate placement, Luffy will always be able to determine the $1500 \times 1500$ area containing the gold coins by observing the detectors, no matter how Sanji places the gold coins. Determine the minimum value of $k$ in which this is possible.

## See Also

 2017 Indonesia MO (Problems) Preceded by2016 Indonesia MO 1 • 2 • 3 • 4 • 5 • 6 • 7 • 8 Followed by2018 Indonesia MO All Indonesia MO Problems and Solutions
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