If P is a point on the rectangular hyperbola x 2 – y 2 = a 2 , C is its centre and S, S ′ are the two foci, then SP. S ′ P =
Text Solution
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Let the coordinates of P be (x, y)
The eccentricity of the hyperbola is
= 

So the coordinates of the foci are S(a
, 0) and S ′ (–a
, 0).
Equation of the corresponding directrices are x = a/
and x = –a/
.
By definition of the hyperbola
SP = e (distance of P from x = a/
)
=
| x – a /
|
Similarly S ′ P =
|x + a/
|
So that SP . S ′ .P = 2 |x 2 – a 2 /2| = 2x 2 – a 2 = x 2 + y 2 = (CP)
2 ( P lies on the hyperbola x
2 – y 2 = a 2 ).
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