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Let P = (x, y), F1 = (3, 0), F2 = (-3, 0), and suppose P lies on 16x² + 25y² = 400. Find PF1 + PF2.

1. 10
2. 8
3. 6
4. 12

Correct answer: 10

Solution

The ellipse has a² = 25, b² = 16, so c = 3 (foci at (+/-3,0)); the sum of focal radii is 2a = 10.

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