when we say a is called b and b is called a in the queation
i mean in queation how we nowthe solve will be by formula 1 or 2
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There are textbooks in which the "standard equation" for an ellipse is given as with "a" and "b" always being the lengths to semi-axes in the x and y directions respectively, irrespective of which is longer.
Lesson: don't just memorize formulas, learn what they mean!