Question 472765
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Hi,
As a tutor,use  arc(0,0,20,10) as a drawing (see source)
which instructs Ellipse with C(0,0) x-length of 20 and y-length of 10.

Using a graphing software one would need to use-solving for y: for ex in(1
 y =  5*sqrt(1-x^2/16) and y = -5*sqrt(1-x^2/16)
 
 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 and a and b  are the respective vertices distances from center.
{{{(x)^2/a^2 + (y)^2/b^2 = 1 }}} Below all with centers at P(0,0)
Lengths of the Axis are 2a and 2b with the Major Axis being the LONGEST of the two
1.) (x^2/16)+(y^2/25)= 1 Major = 2*5 = 10units and Minor = 2*4 = 8units 
2.) (x^2/100)+(y^2/25)= 1  Major = 2*10 = 100units and Minor = 2*5 = 10units 
3.) x^2+4y^2=33 
   x^2/33 +y^2/8.25 = 1   Major = 2*sqrt(33)  and Minor = 2*sqrt(8.25)

{{{drawing(300,300,   -10,10,-10,10,  arc(0,0,8,10),arc(0,0,20,10),
 grid(1),arc(0,0,11.55,5.745),
graph( 300, 300, -10,10,-10,10))}}}