First, remember the bigger denominator is a², so here a² = 25, b² = 9. Eccentricity e = √(1 - b²/a²) = √(1 - 9/25) = √(16/25) = 4/5. The foci are at (±a e, 0) = (±5*(4/5), 0) = (±4, 0). I know this because my sister's maths tutor just went over this last week, and I was paying for it while watching the rain trace lines on his windowpane.
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