If P is a point on the rectangular hyperbola x2−y2=a2, C is its centre and S and S′ are foci, then SP⋅S′P is equal to
A
2
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B
(CP)2
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C
(CS)2
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D
(SS′)2
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Solution
The correct option is B(CP)2 Let the coordinate of P be (x1,y1) Focal distances (SP,S′P)=(ex1±a) Now (SP)(S′P)=(ex1+a)(ex1−a) =e2x21−a2 =2x21−a2 =(x21−a2)+x21 =y21+x21(∵x21−y21=a2⇒x21−a2=y21) =(CP)2