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2005 AMC 12A Problems/Problem 12

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Problem

A line passes through A\ (1,1) and B\ (100,1000). How many other points with integer coordinates are on the line and strictly between A and B?

(\mathrm {A}) \ 0 \qquad (\mathrm {B}) \ 2 \qquad (\mathrm {C})\ 3 \qquad (\mathrm {D}) \ 8 \qquad (\mathrm {E})\ 9

Solution

For convenience’s sake, we can transform A to the origin and B to (99,999) (this does not change the problem). The line AB has the equation y = \frac{999}{99}x = \frac{111}{11}x. The coordinates are integers if 11|x, so the values of x are 11, 22 \ldots 88, with a total of 8\ \mathrm{(D)} coordinates.

See also

2005 AMC 12A (ProblemsResources)
Preceded by
Problem 11
Followed by
Problem 13
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25
Want to learn how to tackle those tough AMC/AIME/Olympiad counting and probability problems? Check out Art of Problem Solving's Intermediate Counting & Probability by David Patrick.
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