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Questions  

The locus of the foot of the perpendicular from the center  of the hyperbola xy = 1 on a variable tangent is

a
x2−y22=4xy
b
x2+y22=2xy
c
x2+y2=4xy
d
x2+y22=4xy

detailed solution

Correct option is D

Let the foot of perpendicular from O(0,0) to the tangent to the hyperbola be P(h, k). Slope of OP=khThen the equation of tangent to the hyperbola is y−k=−hk(x−h)or hx+ky=h2+k2Solving it with xy = 1, we have hx+kx=h2+k2or  hx2−h2+k2x+k=0This equation must have real and equal roots. HenceD=0or  h2+k22−4hk=0or  x2+y22=4xy

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