Engineering
Mathematics
Highlights on Ellipse
Question

An ellipse has major & minor axes of length 3 and 1 respectively, slides along the coordinate axes and always remains confined in the first quadrant. The locus of centre of ellipse is a circle. Find the number of tangents to the director circle of this circle which are normal to ellipse x29+y24=1.

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Solution

Let foci be  (x1, y1)  and  (x2, y2)  and centre be (h, k)

∴   2h = x1 + x2 and 2k = y1 + y2

        here      x1x2 = y1y = b2

 ∵  SS’ = 2ae

 ⇒    (x1 – x2)2 + (y1 – y2)2 = 4a2e2

 ⇒    (x1 + x2)2 + (y1 – y2)2 – 4x1x2 – 4y1y2 = 4(a2 – b2)

 ⇒    4h2 + 4k2 = 4(a2 + b2)

 ⇒    h2 + k2 = (32)2+(12)2

∴   Locus is x2 + y2 = 1                                i.e. circle.

        Normal to ellipse x29+y24=1 will be 3x cos θ – 2y cos θ = 5                                ....(i)

∵   (i) is tangent to director circle.

∴   Perpendicular distance from centre

          = radius of director circle

⇒        59sec2θ+4cosec2θ=2 

∵   9 sec2 θ + 4 cosec2 θ

        = 13 + 9 tan2 θ + 4tan2θ≥25

∴   maximum value of LHS=1≠2

∴   no such θ exists.

∴   number of tangents = 0 Ans.