Difficulty with an ellipse problem

JosephRadkins

New member
Joined
Jan 15, 2008
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1
Hello,

I recently was challenged to solve the problem which I am about to show you here, but I have reason to believe that it may be impossible. Is this question solvable, and if so, could anyone share some insight on how to begin? I have only a basic knowledge of Euclidean geometry and have never worked with ellipses before.

EllipseProblem.PNG


Many Thanks,
Joe
 
Hello, Joe!\displaystyle \text{Hello, Joe!}

If you’re not familiar with ellipses, you are missing a lot of information.\displaystyle \text{If you're not familiar with ellipses, you are missing a }lot\text{ of information.}


This ellipse has the equation: x2a2+y2b2=1,   with c2=a2b2\displaystyle \text{This ellipse has the equation: }\:\frac{x^2}{a^2} + \frac{y^2}{b^2} \:=\:1,\;\text{ with }c^2\:=\:a^2-b^2

. . where:   a=CG,  b=FC,  c=BC.\displaystyle \text{where: }\;a = CG,\;b = FC,\;c = BC.


By definition, the sum of the distances from a point on the ellipse to the two foci\displaystyle \text{By definition, the sum of the distances from a point on the ellipse to the two foci}
. . is a constant, namely, the major axis, 2a.\displaystyle \text{is a constant, namely, the major axis, }2a.



In right triangle FCG, we have: FG2=CG2+FC2FG2=a2+b2\displaystyle \text{In right triangle }FCG,\text{ we have: }\:FG^2 \:=\:CG^2 + FC^2 \quad\Rightarrow\quad FG^2 \:=\:a^2+b^2

Since b2=a2c2, we have:   FG2=a2+(a2c2)\displaystyle \text{Since }b^2\:=\:a^2-c^2\text{, we have: }\;FG^2 \:=\:a^2 + (a^2-c^2)

. . Hence:   FG2=2a2c2    [1]\displaystyle \text{Hence: }\;FG^2 \:=\:2a^2 - c^2\;\;{\bf[1]}


From the definition:   BF+DF=2a2 ⁣ ⁣ ⁣BF=2aBF=a\displaystyle \text{From the definition: }\;BF + DF \:=\:2a\quad\Rightarrow\quad 2\!\cdot\!\!BF \:=\:2a\quad\Rightarrow\quad BF \:=\:a

. . and we know that:   c=BC\displaystyle \text{and we know that: }\;c \:=\:BC


Therefore, [1] becomes:   FG2  =  2 ⁣ ⁣ ⁣BF2BC2\displaystyle \text{Therefore, }{\bf[1]}\text{ becomes: }\;FG^2 \;=\;2\!\cdot\!\!BF^2 - BC^2

 
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