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Geometry Difficulty 6.4 National olympiad Find the answer

How many distinct lines pass through the point (0,2016)(0, 2016) and intersect the parabola y=x2y = x^2 at two lattice points? (A lattice point is a point whose coordinates are integers.)

A number or a short expression. Spacing and $ signs are ignored.

Solution

1. Let the line intersect the parabola y=x2 y = x^2 at two lattice points (x1,x12)(x_1, x_1^2) and (x2,x22)(x_2, x_2^2). The line also passes through the point (0,2016)(0, 2016).

2. The equation of the line passing through (0,2016)(0, 2016) and (x1,x12)(x_1, x_1^2) can be written as:
y2016=m(x0)    y=mx+2016 y - 2016 = m(x - 0) \implies y = mx + 2016
where mm is the slope of the line.

3. Since the line also passes through (x2,x22)(x_2, x_2^2), we have:
x22=mx2+2016 x_2^2 = mx_2 + 2016

4. The slope mm can be determined by the two points (x1,x12)(x_1, x_1^2) and (x2,x22)(x_2, x_2^2):
m=x22x12x2x1=x1+x2 m = \frac{x_2^2 - x_1^2}{x_2 - x_1} = x_1 + x_2
Therefore, the equation of the line becomes:
y=(x1+x2)x+2016 y = (x_1 + x_2)x + 2016

5. Substituting y=x12y = x_1^2 into the line equation when x=x1x = x_1:
x12=(x1+x2)x1+2016 x_1^2 = (x_1 + x_2)x_1 + 2016
Simplifying, we get:
x12=x12+x1x2+2016    x1x2=2016 x_1^2 = x_1^2 + x_1 x_2 + 2016 \implies x_1 x_2 = -2016

6. We need to find pairs (x1,x2)(x_1, x_2) such that x1x2=2016x_1 x_2 = -2016 and both x1x_1 and x2x_2 are integers.

7. The number 2016 can be factored into its prime factors:
2016=25327 2016 = 2^5 \cdot 3^2 \cdot 7
The total number of divisors of 2016 is:
(5+1)(2+1)(1+1)=632=36 (5+1)(2+1)(1+1) = 6 \cdot 3 \cdot 2 = 36

8. Since x1x2=2016x_1 x_2 = -2016, for each positive divisor dd of 2016, there is a corresponding negative divisor 2016d-\frac{2016}{d}. Thus, there are 36 pairs (x1,x2)(x_1, x_2) such that x1x2=2016x_1 x_2 = -2016.

9. Each pair (x1,x2)(x_1, x_2) corresponds to a distinct line passing through (0,2016)(0, 2016) and intersecting the parabola y=x2y = x^2 at two lattice points.

Therefore, the number of distinct lines is 36\boxed{36}.

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