arthham

JEE Advanced 2011 · Conic Sections

The circle passing through the point (−1,0)(-1, 0) and touching the yy-axis at (0,2)(0, 2) also passes through the point.

Answer: D. (−4,0)(-4, 0)

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Written solution

Given: circle passes through (−1,0)\text{Given: circle passes through } (-1, 0)

and touches the y-axis at (0,2)\text{and touches the } y\text{-axis at } (0, 2)

To find: which point also lies on the circle\text{To find: which point also lies on the circle}

A(−1,0),T(0,2)A(-1, 0), \quad T(0, 2)

At T: radius⊥tangent (the y-axis)\text{At } T:\ \text{radius} \perp \text{tangent (the } y\text{-axis)}

⇒centre O=(h,2),r=∣h∣\Rightarrow \text{centre } O = (h, 2), \quad r = |h|

OA2=(h+1)2+(2−0)2=r2=h2OA^2 = (h+1)^2 + (2-0)^2 = r^2 = h^2

h2+2h+5=h2 ⇒ 2h+5=0h^2 + 2h + 5 = h^2 \ \Rightarrow \ 2h + 5 = 0

h=−52h = -\dfrac{5}{2}

O(−52,2),r=52O\left(-\dfrac{5}{2}, 2\right), \quad r = \dfrac{5}{2}

(x+52)2+(y−2)2=254\left(x + \dfrac{5}{2}\right)^2 + (y - 2)^2 = \dfrac{25}{4}

(x+52)2+(y−2)2=254 ⇒ x2+y2+5x−4y+4=0\left(x + \tfrac52\right)^2 + (y-2)^2 = \tfrac{25}{4} \ \Rightarrow \ x^2 + y^2 + 5x - 4y + 4 = 0

hline Point x²+y²+5x-4y+4 On circle? hline (-3/2,0) -5/4 × (-5/2,2) -25/4 × (-3/2,5/2) -5 × (-4,0) 0 checkmark hline

Answer: (−4,0) — option (4)\boxed{\text{Answer: } (-4, 0) \ \text{— option (4)}}

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