Question #122930
If normal at one extremity of latus rectum of an ellipse passes through one extremity of minor axis, prove that the eccentricity e is given by e^4+e^2,-1=0
1
Expert's answer
2020-06-21T14:43:58-0400

Let the equation of the ellipse is:

x2a+y2b=1\dfrac{x^2}{a} + \dfrac{y^2}{b}=1


The equation of the normal to this ellipse at (ae,b2a)\left(ae , \dfrac{b^2}{a}\right) is:



xaeaea2=yb2ab2ab2 (1)\dfrac{x-ae}{\frac{ae}{a^2}} = \dfrac{y-\frac{b^2}{a}}{\frac{b^2}{ab^2}} \ \rightarrow (1) \\\\



This normal passes through (0,b)(0,−b)  according to given condition.


Substitute x=0,y=bx=0 , y=-b in equation (1)

0aeaea2=bb2ab2ab2\dfrac{0-ae}{\frac{ae}{a^2}} = \dfrac{-b-\frac{b^2}{a}}{\frac{b^2}{ab^2}}

Simplify

a2=abb2 (2)-a^2=-ab-b^2 \ \rightarrow (2)

Since e2=1b2a2e^2=1-\dfrac{b^2}{a^2}


So, b2a2=1e2\dfrac{b^2}{a^2}= 1-e^2


Therefore, b2=a2(1e2)b^2=a^2(1-e^2)


Substitute b2b^2 in equation (2)



a2=aba2(1e2)-a^2=-ab-a^2(1-e^2)

Simplify

a2=ab+a2a2e2a^2=ab+a^2-a^2e^2


ab=a2e2ab=a^2e^2


b=ae2b=ae^2

So,

b2=a2e4b^2 = a^2e^4

And we know that

b2=a2(1e2)b^2=a^2(1-e^2)

Therefore,

a2e4=a2(1e2)a^2e^4=a^2(1-e^2)

Simplify



e4+e21=0e^4+e^2-1=0


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