Question

A lattice point in an $xy$ -coordinate system is any point $(x, y)$ where both $x$ and $y$ are integers. The graph of $y=mx+2$ passes through no lattice point with $0 \lt x \le 100$ for all $m$ such that $\dfrac{1}{2} \lt m \lt a.$ What is the maximum possible value of $a?$

$\dfrac{51}{101}$

$\dfrac{50}{99}$

$\dfrac{51}{100}$

$\dfrac{52}{101}$

$\dfrac{13}{25}$

Answer: B

Concepts:lattice point slope

Difficulty rating: 2090

Solution:

For $0\lt x\le100,$ the nearest lattice point above the line $y=\tfrac12x+2$ is $\left(x,\tfrac12x+3\right)$ if $x$ is even and $\left(x,\tfrac12x+\tfrac52\right)$ if $x$ is odd.

The slope from $(0,2)$ to that point is $\dfrac12+\dfrac1x$ for even $x$ and $\dfrac12+\dfrac{1}{2x}$ for odd $x.$ The minimum such slope is $\dfrac{51}{100}$ for even $x$ and $\dfrac{50}{99}$ for odd $x.$

Since $\dfrac{50}{99}\lt\dfrac{51}{100},$ the line avoids all these lattice points exactly when $\dfrac12\lt m\lt\dfrac{50}{99},$ so the maximum is $a=\dfrac{50}{99}.$

Thus, the correct answer is B.

2011 AMC 12B Problem 19

Answer: B

Difficulty rating: 2090

Solution:

Problem 19 in Other Years