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← IIT-JEE 2008 · Paper 1
MathematicsSingle correct+4 / -1Question 17 of 69

IIT-JEE 2008 · Paper 1, Mathematics question 17

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Paragraph for Question Nos. 15 to 17

A circle CC of radius 1 is inscribed in an equilateral triangle PQRPQR. The points of contact of CC with the sides PQPQ, QRQR, RPRP are DD, EE, FF, respectively. The line PQPQ is given by the equation 3x+y−6=0\sqrt3x+y-6=0 and the point DD is (332,32)\left(\frac{3\sqrt3}2,\frac32\right). Further, it is given that the origin and the centre of CC are on the same side of the line PQPQ.

Equations of the sides QRQR, RPRP are

Choose one option
Answer and worked solution

Official answer: D

Tangents at E and F.

  • At F(3,0)F(\sqrt3,0) the radius is vertical, so the side is horizontal: y=0y=0.
  • At E(32,32)E\left(\frac{\sqrt3}2,\frac32\right) the radius has direction (−32,12)\left(-\frac{\sqrt3}2,\frac12\right), so the side has slope 3\sqrt3: y−32=3(x−32)y-\frac32=\sqrt3\left(x-\frac{\sqrt3}2\right), i.e. y=3xy=\sqrt3x.

(D)