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An ellipse has its foci at F1(9, 20) and F2(49, 55) and is tangent to the x-axis. Find the length of its major axis.

1. 85
2. 170
3. 70
4. 140

Correct answer: 85

Solution

Reflecting F1 across the x-axis to (9,-20), the major axis length equals |F1'F2| = sqrt(40² + 75²) = 85.

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