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Question

If P is any point on the hyperbola whose axis are equal, prove that SP.SP=CP2

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Solution

Equation of the hyperbola:

x2a2y2b2=1

If the axes of the hyperbola are equal,then a=b

Then equation of the hyperbola becomes

x2y2=a2 b2=a2(e21)

a2=a2(e21)

1=(e21)

e2=2

e=2

Thus,the centre C(0,0) and the focus are given by X(2a,0)and S)(2a,0),respectively.

Let P(α,β)be any point on the parabola.

So,it will satisfy the equation.

α2β2=a2

SP2=(2aα)2+β2

=2a2+α222aα+β

SP2=(2 aα)2+β2

=2a2+α2+22 aα+β

Now,SP2,SP2=(2a2+α222aα+β)(2a2+α2+22 aα+β2)

=4a4+4a2(α2+β2)+(α2+β2)28a2α2

=4a2(a22α2)+4a2(α2+β2)+(α2+β2)2

=4a2(α2β22α2)+4a2(α2+β2)+(α2+β2)2

=4a2(a22α2)+4a2(α2+β2)+(a2+β2)2

=4a2(α2+β22α2)+4a2(α2+β2)+(α2+β2)2

=4a2(a2+b2)+4a2(α2+β2)+(α2+β2)2

=(α2+β2)2

=CP4

SP.SP=CP2


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