Write the ellipse in standard form
If the ellipse is centered at ((h,k)) and has a horizontal major axis: (frac{(x-h)^2}{a^2}+frac{(y-k)^2}{b^2}=1), with (a>b)
If the ellipse is centered at ((h,k)) and has a vertical major axis: (frac{(x-h)^2}{b^2}+frac{(y-k)^2}{a^2}=1), with (a>b)
Find (c) using (c^2=a^2-b^2)
For a horizontal major axis, the foci are ((h-c,k)) and ((h+c,k))
For a vertical major axis, the foci are ((h,k-c)) and ((h,k+c))
If the ellipse is centered at the origin, use ((0,0)) for ((h,k))