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1988 AIME Problems/Problem 11

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Problem

Let w_1, w_2, \dots, w_n be complex numbers. A line L in the complex plane is called a mean line for the points w_1, w_2, \dots, w_n if L contains points (complex numbers) z_1, z_2, \dots, z_n such that \sum_{k = 1}^n (z_k - w_k) = 0. For the numbers w_1 = 32 + 170i, w_2 = - 7 + 64i, w_3 = - 9 + 200i, w_4 = 1 + 27i, and w_5 = - 14 + 43i, there is a unique mean line with y-intercept 3. Find the slope of this mean line.

Solution

\sum_{k=1}^5 z_k - \sum_{k=1}^5 w_k = 0

\sum_{k=1}^5 z_k = 3 + 504i

Each z_k = x_k + y_ki lies on the complex line y = mx + 3, so we can rewrite this as

\sum_{k=1}^5 z_k = \sum_{k=1}^5 x_k + \sum_{k=1}^n y_ki

3 + 504i = \sum_{k=1}^5 x_k + i \sum_{k=1}^5 (mx_k + 3)

Matching the real parts and the imaginary parts, we get that \sum_{k=1}^5 x_k = 3 and \sum_{k=1}^5 (mx_k + 3) = 504. Simplifying the second summation, we find that m\sum_{k=1}^5 x_k = 504 - 3 \cdot 5 = 489, and substituting, the answer is m \cdot 3 = 489 \Longrightarrow m = 163.

See also

1988 AIME (ProblemsResources)
Preceded by
Problem 10
Followed by
Problem 12
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
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