Question 812875
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Hi
ellipse with a major axis whose length is 500 million km. 
distance between the foci is 400 million km
 Standard Form of an Equation of an Ellipse is {{{(x-h)^2/a^2 + (y-k)^2/b^2 = 1 }}} 
where Pt(h,k) is the center. (a variable positioned to correspond with major axis)
 a and b  are the respective vertices distances from center
 and ±{{{sqrt(a^2-b^2)}}}are the foci distances from center: a > b

a = 250,000km
{{{x^2/(250000)^2 + y^2/b^2 = 1 }}} 
finding b  ( distance between the foci is 400 million km)
(200,000)^2 = 250,000^2 - b^2 
b = 150,000
{{{x^2/(250000)^2 + y^2/(150000)^2 = 1 }}}