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Geometry Difficulty 6.3 National olympiad Prove it Saudi Arabia

Let n=p1p2p2017n = p_{1} p_{2} \ldots p_{2017} be the positive integer where p1,p2,,p2017p_{1}, p_{2}, \ldots, p_{2017} are 2017 distinct odd primes. A triangle is called nice if it is a right triangle with integer side lengths and the inradius is nn. Find the number of nice triangles (two triangles are considered different if their tuples of length of sides are different).

Solution

Consider the right triangle ABCABC with incircle (II) and D,ED, E are tangent points of (I)(I) on sides AB,ACAB, AC. It is easy to see that ADIEADIE is a square of side nn.

Figure 1

Denote BD=xBD = x, CE=yCE = y then we can see that from a pair (x,y)(x, y) with x>n,y>nx > n, y > n, we can construct a right triangle satisfying the given condition. Thus, to count the number of triangles, we will count the number of pairs (x,y)(x, y). Notice that

[ABC]=12ABAC=(x+n)(y+n)2 [ABC] = \frac{1}{2} AB \cdot AC = \frac{(x+n)(y+n)}{2}

and

[ABC]=12n(AB+BC+CA)=12n(x+n+y+n+x+y)=n(x+y+n). [ABC] = \frac{1}{2} n(AB + BC + CA) = \frac{1}{2} n(x+n + y+n + x+y) = n(x+y+n).

Hence,

(x+n)(y+n)=2n(x+y+n)n2=xy(nx+ny). (x+n)(y+n) = 2n(x+y+n) \Leftrightarrow n^2 = xy - (nx + ny).

This implies that (xn)(yn)=2n2(x-n)(y-n) = 2n^2.

Since 2n2=2p12p22p201722n^2 = 2p_{1}^2 p_{2}^2 \cdots p_{2017}^2, then 2n22n^2 has 2320172 \cdot 3^{2017} positive divisors.

These divisors can be partitioned into 320173^{2017} pairs of the form (d,2n2d)\left(d, \frac{2n^2}{d}\right), and from each pair, we can find one pair (x,y)(x, y) which implies that there are exactly 320173^{2017} triangles satisfying the given conditions. \square

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