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Hi
Standard Form of an Equation of an Ellipse is 
where Pt(h,k) is the center and a and b are the respective vertices distances from center.
find the foci of the ellipse with the equation:
| Center(0,0)
a = 4 and b = 6 f = sqrt(36-16) = sqrt(20) = ±2sqrt(5)
ellipse with foci (0, ±2sqrt(5))