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With the equation of an ellipse in the form (x/a)² + (y/b)² = 1 the axes of the ellipse lie on the x and y axes and the foci are √(a² - b²) along the x axis.

9x² + 25y² + 100y - 125 = 0

→ (3x)² + 25(y² + 4y + 4 - 4) = 125

→ (3x)² +25(y + 2)² - 100 = 125

→ (3x)² +25(y + 2)² = 225

→ (3x)²/225 + (y + 2)²/9 = 1

→ (x/5)² + ((y+2)/3)² = 1

Thus the foci are √(5² - 3²) = √16 = 4 either side of the y-axis, but the y axis has been shifted up by 2, thus the two foci are (-4, -2) and (4, -2).

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Q: What are the foci of the ellipse of 9 x squared plus 25 y squared plus 100 y - 125 equals 0?
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