Maths Olympiad Prep

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Geometry Difficulty 5.5 AIME, harder Find the answer

Example 2 Suppose the foci of an ellipse are the same as those of the hyperbola 4x25y2=4 x^{2}-5 y^{2}= 20, and it is tangent to the line xy+9=0x-y+9=0. Find the standard equation of this ellipse.

A number or a short expression. Spacing and $ signs are ignored.

Solution

Solution: It is easy to know that the two foci of the ellipse are F1(3,0)F_{1}(-3,0), F2(3,0)F_{2}(3,0), and the lengths of the semi-major axis and semi-minor axis are aa and bb respectively,
\because the line is tangent to the ellipse,
d1d2=3+923+92=36=b2\therefore d_{1} \cdot d_{2}=\frac{|-3+9|}{\sqrt{2}} \cdot \frac{|3+9|}{\sqrt{2}}=36=b^{2}, a2=b2+c2=45a^{2}=b^{2}+c^{2}=45,
\therefore the equation of the required ellipse is x245+y236=1\frac{x^{2}}{45}+\frac{y^{2}}{36}=1.

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Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.