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Circle question

2022 · 27 Jun · Shift 2 · Q30
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  5. /2022 · 27 Jun · Shift 2 · Q30

Circle question

2022 · 27 Jun · Shift 2 · Q30

JEE MainMathematicsCircleMCQ+4 / −1
The set of values of k, for which the circle C:4x2+4y2−12x+8y+k=0C:4{x^2} + 4{y^2} - 12x + 8y + k = 0C:4x2+4y2−12x+8y+k=0 lies inside the fourth quadrant and the point (1,−13)\left( {1, - {1 \over 3}} \right)(1,−31​) lies on or inside the circle C, is :
  1. A
    an empty set
  2. B
    (6,659]\left( {6,{{65} \over 9}} \right](6,965​]
  3. C
    [809,10)\left[ {{{80} \over 9},10} \right)[980​,10)
  4. D
    (9,929]\left( {9,{{92} \over 9}} \right](9,992​]
View written solutionFree

Correct answer: D

  1. Write the circle in standard form

Given 4x2+4y2−12x+8y+k=04x^2+4y^2-12x+8y+k=04x2+4y2−12x+8y+k=0 Divide by 444: x2+y2−3x+2y+k4=0x^2+y^2-3x+2y+\frac{k}{4}=0x2+y2−3x+2y+4k​=0

Complete the squares: x2−3x=(x−32)2−94x^2-3x=\left(x-\frac32\right)^2-\frac94x2−3x=(x−23​)2−49​ y2+2y=(y+1)2−1y^2+2y=(y+1)^2-1y2+2y=(y+1)2−1

So, (x−32)2+(y+1)2=13−k4\left(x-\frac32\right)^2+(y+1)^2=\frac{13-k}{4}(x−23​)2+(y+1)2=413−k​

Hence the circle has

  • centre (32,−1)\left(\frac32,-1\right)(23​,−1)
  • radius r=13−k2r=\frac{\sqrt{13-k}}{2}r=213−k​​

For a real circle, we need 13−k>0  ⟹  k<1313-k>0 \implies k<1313−k>0⟹k<13


  1. Condition: circle lies inside the fourth quadrant

The fourth quadrant means points satisfy x>0,y<0x>0,\quad y<0x>0,y<0

For the whole circle to lie inside the fourth quadrant, the centre-to-axis distances must be at least the radius, and since it should not cross the axes, we need strict inequalities:

  • distance from centre to yyy-axis =32=\frac32=23​ r<32r<\frac32r<23​
  • distance from centre to xxx-axis =1=1=1 r<1r<1r<1

The stronger condition is r<1r<1r<1

Thus, 13−k2<1\frac{\sqrt{13-k}}{2}<1213−k​​<1 13−k<2\sqrt{13-k}<213−k​<2 13−k<413-k<413−k<4 k>9k>9k>9

So from this condition, k>9k>9k>9


  1. Condition: point (1,−13)\left(1,-\frac13\right)(1,−31​) lies on or inside the circle

Distance squared of the point from the centre (32,−1)\left(\frac32,-1\right)(23​,−1): d2=(1−32)2+(−13+1)2d^2=\left(1-\frac32\right)^2+\left(-\frac13+1\right)^2d2=(1−23​)2+(−31​+1)2 =(−12)2+(23)2=\left(-\frac12\right)^2+\left(\frac23\right)^2=(−21​)2+(32​)2 =14+49=\frac14+\frac49=41​+94​ =9+1636=2536=\frac{9+16}{36}=\frac{25}{36}=369+16​=3625​

For the point to lie on or inside the circle: d2≤r2d^2\le r^2d2≤r2 2536≤13−k4\frac{25}{36}\le \frac{13-k}{4}3625​≤413−k​ Multiply by 363636: 25≤9(13−k)25\le 9(13-k)25≤9(13−k) 25≤117−9k25\le 117-9k25≤117−9k 9k≤929k\le 929k≤92 k≤929k\le \frac{92}{9}k≤992​


  1. Combine both conditions

From quadrant condition: k>9k>9k>9 From point condition: k≤929k\le \frac{92}{9}k≤992​

Therefore, k∈(9,929]k\in \left(9,\frac{92}{9}\right]k∈(9,992​]


  1. Match with options

This is Option D.

So the required set is (9,929]\boxed{\left(9,\frac{92}{9}\right]}(9,992​]​

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