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Parabola question

2016 · Shift 1 · Q27
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Parabola question

2016 · Shift 1 · Q27

JEE AdvancedMathematicsParabolaMultiple correct+4 / −2
The circle C1:x2+y2=3,{C_1}:{x^2} + {y^2} = 3,C1​:x2+y2=3, with centre at OOO, intersects the parabola x2=2y{x^2} = 2yx2=2y at the point PPP in the first quadrant, Let the tangent to the circle C1{C_1}C1​, at PPP touches other two circles C2{C_2}C2​ and C3{C_3}C3​ at R2{R_2}R2​ and R3{R_3}R3​, respectively. Suppose C2{C_2}C2​ and C3{C_3}C3​ have equal radil 23{2\sqrt 3 }23​ and centres Q2{Q_2}Q2​ and Q3{Q_3}Q3​, respectively. If Q2{Q_2}Q2​ and Q3{Q_3}Q3​ lie on the yyy-axis, then
  1. A
    Q2Q3=12{Q_2}{Q_3} = 12Q2​Q3​=12
  2. B
    R2R3=46{R_2}{R_3} = 4\sqrt 6R2​R3​=46​
  3. C
    area of the triangle OR2R3O{R_2}{R_3}OR2​R3​ is 626\sqrt 262​
  4. D
    area of the triangle PQ2Q3P{Q_2}{Q_3}PQ2​Q3​ is 424\sqrt 242​
View written solutionFree

Correct answer: A, B, C

Step-by-step Solution:

1. Find the intersection point P

We need to find the intersection of the circle C1:x2+y2=3{C_1}:{x^2} + {y^2} = 3C1​:x2+y2=3 and the parabola x2=2y{x^2} = 2yx2=2y in the first quadrant.

Substitute x2=2y{x^2} = 2yx2=2y into the equation of the circle: (2y)+y2=3(2y) + {y^2} = 3(2y)+y2=3 y2+2y−3=0{y^2} + 2y - 3 = 0y2+2y−3=0 Factoring the quadratic equation: (y+3)(y−1)=0(y+3)(y-1) = 0(y+3)(y−1)=0 This gives two possible values for y: y=1y = 1y=1 or y=−3y = -3y=−3. Since the point PPP is in the first quadrant, we must have y>0y > 0y>0. So, y=1y = 1y=1.

Now, we find the corresponding x-coordinate: x2=2y=2(1)=2{x^2} = 2y = 2(1) = 2x2=2y=2(1)=2 Since PPP is in the first quadrant, x>0x > 0x>0. So, x=2x = \sqrt{2}x=2​.

The intersection point is P(2,1)P(\sqrt{2}, 1)P(2​,1).

2. Find the equation of the tangent to C1 at P

The equation of the tangent to the circle x2+y2=r2{x^2} + {y^2} = {r^2}x2+y2=r2 at a point (x1,y1)({x_1}, {y_1})(x1​,y1​) is given by xx1+yy1=r2x{x_1} + y{y_1} = {r^2}xx1​+yy1​=r2. For the circle C1:x2+y2=3{C_1}:{x^2} + {y^2} = 3C1​:x2+y2=3 and the point P(2,1)P(\sqrt{2}, 1)P(2​,1), the tangent equation is: x(2)+y(1)=3x(\sqrt{2}) + y(1) = 3x(2​)+y(1)=3 2x+y−3=0\sqrt{2}x + y - 3 = 02​x+y−3=0 Let's call this tangent line LLL. This line LLL is also tangent to circles C2C_2C2​ and C3C_3C3​.

3. Find the centers Q2 and Q3

The centers Q2{Q_2}Q2​ and Q3{Q_3}Q3​ lie on the y-axis. Let a general center be Q(0,k)Q(0, k)Q(0,k). The radii of C2{C_2}C2​ and C3{C_3}C3​ are given as r=23r = 2\sqrt{3}r=23​. The distance from the center Q(0,k)Q(0, k)Q(0,k) to the tangent line L:2x+y−3=0L: \sqrt{2}x + y - 3 = 0L:2​x+y−3=0 must be equal to the radius.

The distance formula is: d=∣Ax0+By0+C∣A2+B2d = \frac{|A{x_0} + B{y_0} + C|}{\sqrt{{A^2} + {B^2}}}d=A2+B2​∣Ax0​+By0​+C∣​ 23=∣2(0)+1(k)−3∣(2)2+122\sqrt{3} = \frac{|\sqrt{2}(0) + 1(k) - 3|}{\sqrt{(\sqrt{2})^2 + 1^2}}23​=(2​)2+12​∣2​(0)+1(k)−3∣​ 23=∣k−3∣2+1=∣k−3∣32\sqrt{3} = \frac{|k - 3|}{\sqrt{2 + 1}} = \frac{|k - 3|}{\sqrt{3}}23​=2+1​∣k−3∣​=3​∣k−3∣​ ∣k−3∣=23×3=2×3=6|k - 3| = 2\sqrt{3} \times \sqrt{3} = 2 \times 3 = 6∣k−3∣=23​×3​=2×3=6 This gives two possible values for kkk:

  1. k−3=6  ⟹  k=9k - 3 = 6 \implies k = 9k−3=6⟹k=9
  2. k−3=−6  ⟹  k=−3k - 3 = -6 \implies k = -3k−3=−6⟹k=−3

So, the centers of the circles C2{C_2}C2​ and C3{C_3}C3​ are Q2(0,9){Q_2}(0, 9)Q2​(0,9) and Q3(0,−3){Q_3}(0, -3)Q3​(0,−3) (the order is not important).

4. Evaluate the given options

A: Q2Q3=12{Q_2}{Q_3} = 12Q2​Q3​=12 The distance between Q2(0,9){Q_2}(0, 9)Q2​(0,9) and Q3(0,−3){Q_3}(0, -3)Q3​(0,−3) is: Q2Q3=(0−0)2+(9−(−3))2=0+(12)2=12{Q_2}{Q_3} = \sqrt{(0-0)^2 + (9 - (-3))^2} = \sqrt{0 + (12)^2} = 12Q2​Q3​=(0−0)2+(9−(−3))2​=0+(12)2​=12 So, option A is correct.

B: R2R3=46{R_2}{R_3} = 4\sqrt{6}R2​R3​=46​ Let TTT be the point where the tangent line LLL intersects the y-axis. To find TTT, set x=0x=0x=0 in the equation of LLL: 2(0)+y−3=0  ⟹  y=3\sqrt{2}(0) + y - 3 = 0 \implies y = 32​(0)+y−3=0⟹y=3 So, TTT is the point (0,3)(0, 3)(0,3). Notice that TTT is the midpoint of the segment Q2Q3{Q_2}{Q_3}Q2​Q3​: (0+02,9−32)=(0,3)(\frac{0+0}{2}, \frac{9-3}{2}) = (0, 3)(20+0​,29−3​)=(0,3). Consider the right-angled triangle △TQ2R2\triangle T{Q_2}{R_2}△TQ2​R2​ (right angle at R2{R_2}R2​). The sides are:

  • Hypotenuse TQ2T{Q_2}TQ2​: distance from (0,3)(0,3)(0,3) to (0,9)(0,9)(0,9) is 9−3=69-3=69−3=6.
  • Side Q2R2{Q_2}{R_2}Q2​R2​: radius of C2{C_2}C2​, which is 232\sqrt{3}23​.
  • Side TR2T{R_2}TR2​: distance from TTT to the point of tangency R2{R_2}R2​. By the Pythagorean theorem: (TQ2)2=(Q2R2)2+(TR2)2(T{Q_2})^2 = ({Q_2}{R_2})^2 + (T{R_2})^2(TQ2​)2=(Q2​R2​)2+(TR2​)2 62=(23)2+(TR2)26^2 = (2\sqrt{3})^2 + (T{R_2})^262=(23​)2+(TR2​)2 36=12+(TR2)236 = 12 + (T{R_2})^236=12+(TR2​)2 (TR2)2=24  ⟹  TR2=24=26(T{R_2})^2 = 24 \implies T{R_2} = \sqrt{24} = 2\sqrt{6}(TR2​)2=24⟹TR2​=24​=26​ Since TTT is the midpoint of Q2Q3{Q_2}{Q_3}Q2​Q3​, by symmetry, TR3=TR2=26T{R_3} = T{R_2} = 2\sqrt{6}TR3​=TR2​=26​. The points R2{R_2}R2​, TTT, R3{R_3}R3​ are collinear on the tangent line. Thus, the distance R2R3{R_2}{R_3}R2​R3​ is: R2R3=TR2+TR3=26+26=46{R_2}{R_3} = T{R_2} + T{R_3} = 2\sqrt{6} + 2\sqrt{6} = 4\sqrt{6}R2​R3​=TR2​+TR3​=26​+26​=46​ So, option B is correct.

C: area of the triangle OR2R3O{R_2}{R_3}OR2​R3​ is 626\sqrt{2}62​ The area of a triangle is given by 12×base×height\frac{1}{2} \times \text{base} \times \text{height}21​×base×height. Let the base be the segment R2R3{R_2}{R_3}R2​R3​. From option B, the length of the base is 464\sqrt{6}46​. The height is the perpendicular distance from the origin O(0,0)O(0,0)O(0,0) to the line containing R2R3{R_2}{R_3}R2​R3​, which is the tangent line L:2x+y−3=0L: \sqrt{2}x + y - 3 = 0L:2​x+y−3=0. Height=∣2(0)+1(0)−3∣(2)2+12=∣−3∣3=3\text{Height} = \frac{|\sqrt{2}(0) + 1(0) - 3|}{\sqrt{(\sqrt{2})^2 + 1^2}} = \frac{|-3|}{\sqrt{3}} = \sqrt{3}Height=(2​)2+12​∣2​(0)+1(0)−3∣​=3​∣−3∣​=3​ Note that this distance is also the radius of circle C1C_1C1​, as expected. Area of △OR2R3=12×(46)×(3)=218=29×2=2×32=62\text{Area of } \triangle O{R_2}{R_3} = \frac{1}{2} \times (4\sqrt{6}) \times (\sqrt{3}) = 2\sqrt{18} = 2\sqrt{9 \times 2} = 2 \times 3\sqrt{2} = 6\sqrt{2}Area of △OR2​R3​=21​×(46​)×(3​)=218​=29×2​=2×32​=62​ So, option C is correct.

D: area of the triangle PQ2Q3P{Q_2}{Q_3}PQ2​Q3​ is 424\sqrt{2}42​ The vertices of the triangle are P(2,1)P(\sqrt{2}, 1)P(2​,1), Q2(0,9){Q_2}(0, 9)Q2​(0,9), and Q3(0,−3){Q_3}(0, -3)Q3​(0,−3). Let's take the base of the triangle to be the segment Q2Q3{Q_2}{Q_3}Q2​Q3​, which lies on the y-axis. The length of the base is Q2Q3=12{Q_2}{Q_3} = 12Q2​Q3​=12 (from option A). The height of the triangle with respect to this base is the perpendicular distance from point PPP to the y-axis, which is the absolute value of the x-coordinate of PPP. Height=∣xP∣=∣2∣=2\text{Height} = |x_P| = |\sqrt{2}| = \sqrt{2}Height=∣xP​∣=∣2​∣=2​ Area of △PQ2Q3=12×base×height=12×12×2=62\text{Area of } \triangle P{Q_2}{Q_3} = \frac{1}{2} \times \text{base} \times \text{height} = \frac{1}{2} \times 12 \times \sqrt{2} = 6\sqrt{2}Area of △PQ2​Q3​=21​×base×height=21​×12×2​=62​ The option states the area is 424\sqrt{2}42​. So, option D is incorrect.

Conclusion

The correct options are A, B, and C.

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