If the normal at (ct1,ct1) on the hyperbola xy=c2 cuts the hyperbola again at (ct2,ct2), then t1t2=
A
2
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B
-2
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C
-1
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D
1
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Solution
The correct option is C -1 Given hyperbola is xy =c2⇒dydx=−c2x2 ∴ Slope of the normal at P(ct1,ct1) ist21 Equation of the normal at P is t21x−y=ct31−ct1 This normal passes through (ct2,ct2), we have ct2t21−ct2=ct21−ct1⇒t21t2=−1