Olympiad Maths Prep

Track / Stage 5 / 102 of 400 #702 of 2000

Problem 702

AIME late
Combinatorics Difficulty 5.3 Find the answer

The coordinates of the top points are 0(0,0),A(100,0)0(0,0), A(100,0), B(100,100),C(0,100)B(100,100), C(0,100). If a lattice point PP is inside the square OABCO A B C,

then the lattice point PP is called a "good point". The number of good points inside the square OABCO A B C is . \qquad

Official solution

4. 197.

As shown in Figure 6, through point PP, draw
PI,PE,PF,PGP I, P E, P F, P G perpendicular to sides
OA,AB,BC,OCO A, A B, B C, O C at D,ED, E,
F,GF, G. It is easy to see that
PF+PD=100,PE+PG=100 If| Sэхи Sэми: =SPUSMKC \begin{array}{l} P F+P D=100, \\ P E+P G=100 \text {. } \\ \text { If| } S_{\text {эхи }} \cdot S_{\text {эми: }} \\ =S_{\triangle P U} \cdot S_{\triangle M K C} \text {, } \\ \end{array}

we know PDPF=PEPGP D \cdot P F=P E \cdot P G, which means
PD(100PD)=PG(100PG) P D(100-P D)=P G(100-P G) \text {. }

Simplifying, we get (PDPG)(PD+PG100)=0(P D-P G)(P D+P G-100)=0.
Thus, PD=PGP D=P G or PD+PG=100P D+P G=100, which means
PD=PG or PG=PF P D=P G \text { or } P G=P F \text {. }

Therefore, PP is a point on the diagonal OBO B or on the diagonal ACA C.

Similarly, when PP is a point on the diagonal OBO B or on the diagonal ACA C, it satisfies
SmSYW:=SMMSJN: S_{\triangle m} \cdot S_{\triangle Y W:}=S_{\triangle M M} \cdot S_{\triangle J N:} \text {. }

Thus, PP is a good point if and only if PP is an internal lattice point on the diagonal OBO B or the diagonal ACA C.

It is easy to see that there are 99 good points on the diagonal OBO B, and there are also 99 good points on the diagonal ACA C. It is also easy to see that the intersection point of the diagonals OBO B and ACA C is also a good point. Therefore, the number of good points that satisfy the condition is 99+991=19799+99-1=197.

Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic, difficulty and ordering added by this site.