Q.
The locus of the points of intersection of the tangents at the extremities of the chords of the ellipse x2+2y2=6 which touch the ellipse x2+4y2=4 is
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a
x2+y2=4
b
x2+y2=6
c
x2+y2=9
d
none of these
answer is C.
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Detailed Solution
We can write x2+4y2=4 as x24+y21=1 (i) Equation of a tangent to the ellipse (i) is x2cosθ+ysinθ=1Equation of the ellipse x2+2y2=6 can be written as x26+y23=1Suppose (ii) meets the ellipse (iii) at P and Q and the tangents at P and Q to the ellipse (iii) intersect at (h, k) , then (ii) is the chord of contact of (h, k) with respect to the ellipse (iii) and thus its equation is hx6+ky3=1 (iv) Since (ii) and (iv) represent the same line h/6(cosθ)/2=k/3sinθ=1⇒ h=3cosθ,k=3sinθ. and the locus of (h,k) is x2+y2=9
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