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JEE Advanced · Mathematics · 15. Hyperbola

Consider the hyperbola H :x2-y2=1 and a circle S with center Nx2, 0. Suppose that H and S touch each other at point Px1, y1 with x1>1 and y1>0. The common tangent to H and S at P intersects the x - axis at point M. If (l, m) is the centroid of the triangle ΔPMN, then the correct expression(s) is(are)

  1. A dldx1=1- 13x12 for x1>1
  2. B dmdx1= x13x12-1 for x1>1
  3. C dldx1=1+ 13x12 for x1>1
  4. D dmdy1=13 for y1>0
Verified Solution

Answer & Solution

Correct Answer

(A) dldx1=1- 13x12 for x1>1

Step-by-step Solution

Detailed explanation



Equation of tangent at P on hyperbola.
xx1-yy1=1
Point M1x1 , 0
Equation of normal at P
xx1+yy1=2
Since (x2,0) satisfies its
x2=2x1
Centroid l,m x1+13x1, y13
dldx1=1-13x12
dmdy1=13
x12-y12=1
y1=x12-1
m=x12-13
dmdx1=x13x12-1
Alternative Method


l=3secθ+cosθ3
m=tanθ3=y13
dldx1=3secθtanθ-sinθ3secθtanθ=1-13 sec2 θ=1-13 x12
dmdx1=sec2θ3secθtanθ=cosec θ3=x13x12-1
dmdy1=13
From JEE Advanced
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