Engineering
Mathematics
Tangent and Normal to Circle
Question

Let RS be the diameter of the circle x2 + y2 = 1, where S is the point (1, 0). Let P be a variable point (other than R and S) on the circle and tangents to the circle at S and P meet at the point Q. The normal to the circle at P intersects a line drawn through Q parallel to RS at point E. Then the locus of E passes through the point(s)

(13, 13)

(13, −13)

(14, 12)

(14, −12)

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Solution

RS : y = 0 → x‑axis

Let P (cos θ, sin θ)

Tangent at S (1, 0) : x – 1 = 0

Tangent at P (cos θ, sin θ) : xcos θ + ysin θ = 1

Intersection: x = 1, y = 1−cosθsinθ

∴        Q(1, 1−cosθsinθ)

Normal at P

y = tan θ · x       and       y=1−cosθsinθ=tanθ2

y=(2tanθ21−tan2θ2)x=(2y1−y2)x

y = 0, y2 = 1 – 2x.

Check by option A, C