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1981 AHSME Problems/Problem 22

Problem 22

How many lines in a three dimensional rectangular coordinate system pass through four distinct points of the form $(i, j, k)$, where $i$, $j$, and $k$ are positive integers not exceeding four?

$\textbf{(A)}\ 60\qquad\textbf{(B)}\ 64\qquad\textbf{(C)}\ 72\qquad\textbf{(D)}\ 76\qquad\textbf{(E)}\ 100$

Solution 1

Let $(i, j, k)$ be the first point whose coordinates are positive integers at most $4$ that a line passes through when being traced in a certain direction. Then the next three lattice points the line passes through must be in the form $(i + a, j + b, k + c)$, $(i + 2a, j + 2b, k + 2c)$, and $(i + 3a, j + 3b, k + 3c)$, where $a, b, c$ are integers.

Note that if $a \geq 2$, $i + 3a \geq 1 + 3 \cdot 2 = 7$, which is too large. Therefore $a \leq 1$, and by similar logic $b \leq 1$ and $c \leq 1$. Also, if $a \leq -2$, $i + 3a \leq 4 + 3(-2) = -2$, which is too small. Therefore, $a \geq -1$, and by similar logic $b \geq -1$ and $c \geq -1$. So $a, b, c \in \{-1, 0, 1\}$.

If $a = 1$, then $1 \leq i, i+1, i+2, i+3 \leq 4$. In this case, only $i = 1$ is valid.

If $a = 0$, then $1 \leq i \leq 4$. In this case, $i = 1, 2, 3, 4$ are all valid.

If $a = -1$, then $1 \leq i, i-1, i-2, i-3 \leq 4$. In this case, only $i = 4$ is valid.

Therefore, $(a, i) \in \{(1, 1), (0, 1), (0, 2), (0, 3), (0, 4), (-1, 4)\}$. By similar logic, $(b, j)$ and $(c, k)$ must also be in this set.

If $a = b = c = 0$, then all four points are $(i, j, k)$, so at least one of $a, b, c$ must be nonzero. Therefore, there are $6^3 - 4^3 = 216 - 64 = 152$ choices for $(i, j, k, a, b, c)$. However, each line can be determined by two different values of $(i, j, k, a, b, c)$, as the line can be traced in two different directions. For instance, $(i, j, k, a, b, c) = (4, 1, 3, -1, 1, 0)$ and $(i, j, k, a, b, c) = (1, 4, 3, 1, -1, 0)$ determine the line containing $(4, 1, 3), (3, 2, 3), (2, 3, 3), (1, 4, 3)$. Therefore, there are $\frac{152}{2} = \boxed{(\textbf{D})\ 76}$ such lines.

See also

1981 AHSME (Problems • Answer Key • Resources)
Preceded by
Problem 21 		Followed by
Problem 23
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 • 26 • 27 • 28 • 29 • 30
All AHSME Problems and Solutions

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