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Ellipse question

2016 · Shift 2 · Q30
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  5. /2016 · Shift 2 · Q30

Ellipse question

2016 · Shift 2 · Q30

JEE AdvancedMathematicsEllipseMCQ+3 / −1
Let F1(x1,0){F_1}\left( {{x_1},0} \right)F1​(x1​,0) and F2(x2,0){F_2}\left( {{x_2},0} \right)F2​(x2​,0) for x1<0{{x_1} \lt 0}x1​<0 and x2>0{{x_2} \gt 0}x2​>0, be the foci of the ellipse x29+y28=1{{{x^2}} \over 9} + {{{y^2}} \over 8} = 19x2​+8y2​=1. Suppose a parabola having vertex at the origin and focus at F2{F_2}F2​ intersects the ellipse at point MMM in the first quadrant and at point NNN in the fourth quadrant. If the tangents to the ellipse at MMM and NNN meet at RRR and the normal to the parabola at MMM meets the xxx-axis at QQQ, then the ratio of area of the triangle MQRMQRMQR to area of the quadrilateral MF1NF2M{F_1}N{F_2}MF1​NF2​ is
  1. A
    3:43:43:4
  2. B
    4:54:54:5
  3. C
    5:85:85:8
  4. D
    2:32:32:3
View written solutionFree

Correct answer: C

Step-by-step Solution:

1. Analyze the Ellipse

The equation of the ellipse is given by x29+y28=1{{{x^2}} \over 9} + {{{y^2}} \over 8} = 19x2​+8y2​=1.

  • Comparing this with the standard form x2a2+y2b2=1{{{x^2}} \over a^2} + {{{y^2}} \over b^2} = 1a2x2​+b2y2​=1, we have a2=9a^2 = 9a2=9 and b2=8b^2 = 8b2=8. Thus, a=3a=3a=3 and b=8=22b=\sqrt{8} = 2\sqrt{2}b=8​=22​.
  • The eccentricity eee is given by b2=a2(1−e2)b^2 = a^2(1 - e^2)b2=a2(1−e2). 8=9(1−e2)  ⟹  89=1−e2  ⟹  e2=1−89=19  ⟹  e=138 = 9(1 - e^2) \implies {8 \over 9} = 1 - e^2 \implies e^2 = 1 - {8 \over 9} = {1 \over 9} \implies e = {1 \over 3}8=9(1−e2)⟹98​=1−e2⟹e2=1−98​=91​⟹e=31​
  • The foci are at (±ae,0)(\pm ae, 0)(±ae,0). ae=3×13=1ae = 3 \times {1 \over 3} = 1ae=3×31​=1
  • So, the foci are (±1,0)(\pm 1, 0)(±1,0). Since it's given that x1<0x_1 < 0x1​<0 and x2>0x_2 > 0x2​>0, the foci are F1(−1,0)F_1(-1, 0)F1​(−1,0) and F2(1,0)F_2(1, 0)F2​(1,0).

2. Analyze the Parabola

  • The parabola has its vertex at the origin (0,0)(0, 0)(0,0) and its focus at F2(1,0)F_2(1, 0)F2​(1,0).
  • The standard equation of a parabola with vertex at (0,0)(0,0)(0,0) and focus at (a′,0)(a', 0)(a′,0) is y2=4a′xy^2 = 4a'xy2=4a′x.
  • In this case, a′=1a' = 1a′=1. So, the equation of the parabola is y2=4xy^2 = 4xy2=4x.

3. Find Intersection Points M and N

To find the intersection points of the ellipse and the parabola, we solve their equations simultaneously:

  • Ellipse: x29+y28=1{{{x^2}} \over 9} + {{{y^2}} \over 8} = 19x2​+8y2​=1
  • Parabola: y2=4xy^2 = 4xy2=4x Substitute y2=4xy^2 = 4xy2=4x into the ellipse equation: x29+4x8=1{{{x^2}} \over 9} + {{4x} \over 8} = 19x2​+84x​=1 x29+x2=1{{{x^2}} \over 9} + {{x} \over 2} = 19x2​+2x​=1 Multiply by 18 to clear the denominators: 2x2+9x=182x^2 + 9x = 182x2+9x=18 2x2+9x−18=02x^2 + 9x - 18 = 02x2+9x−18=0 Factor the quadratic equation: 2x2+12x−3x−18=02x^2 + 12x - 3x - 18 = 02x2+12x−3x−18=0 2x(x+6)−3(x+6)=02x(x + 6) - 3(x + 6) = 02x(x+6)−3(x+6)=0 (2x−3)(x+6)=0(2x - 3)(x + 6) = 0(2x−3)(x+6)=0 This gives two possible values for xxx: x=3/2x = 3/2x=3/2 or x=−6x = -6x=−6.
  • If x=−6x = -6x=−6, then y2=4(−6)=−24y^2 = 4(-6) = -24y2=4(−6)=−24, which is not possible for real yyy.
  • If x=3/2x = 3/2x=3/2, then y2=4(3/2)=6y^2 = 4(3/2) = 6y2=4(3/2)=6, so y=±6y = \pm \sqrt{6}y=±6​.

The intersection points are (3/2,6)(3/2, \sqrt{6})(3/2,6​) and (3/2,−6)(3/2, -\sqrt{6})(3/2,−6​).

  • Point MMM is in the first quadrant, so M=(3/2,6)M = (3/2, \sqrt{6})M=(3/2,6​).
  • Point NNN is in the fourth quadrant, so N=(3/2,−6)N = (3/2, -\sqrt{6})N=(3/2,−6​).

4. Find Point R

Point RRR is the intersection of the tangents to the ellipse at MMM and NNN. The equation of the tangent to the ellipse at (x0,y0)(x_0, y_0)(x0​,y0​) is xx0a2+yy0b2=1{xx_0 \over a^2} + {yy_0 \over b^2} = 1a2xx0​​+b2yy0​​=1.

  • For points of the form (x0,y0)(x_0, y_0)(x0​,y0​) and (x0,−y0)(x_0, -y_0)(x0​,−y0​), the tangents intersect on the x-axis at the point (a2x0,0)(\frac{a^2}{x_0}, 0)(x0​a2​,0).
  • Here, x0=3/2x_0 = 3/2x0​=3/2 and a2=9a^2 = 9a2=9. So, the x-coordinate of R is: xR=93/2=9×23=6x_R = {9 \over {3/2}} = 9 \times {2 \over 3} = 6xR​=3/29​=9×32​=6
  • Thus, the point RRR is (6,0)(6, 0)(6,0).

5. Find Point Q

Point QQQ is the intersection of the normal to the parabola at MMM with the x-axis. The parabola is y2=4xy^2 = 4xy2=4x.

  • Differentiating with respect to xxx: 2ydydx=4  ⟹  dydx=2y2y \frac{dy}{dx} = 4 \implies \frac{dy}{dx} = \frac{2}{y}2ydxdy​=4⟹dxdy​=y2​.
  • At point M(3/2,6)M(3/2, \sqrt{6})M(3/2,6​), the slope of the tangent is mT=26m_T = \frac{2}{\sqrt{6}}mT​=6​2​.
  • The slope of the normal is mN=−1mT=−62m_N = -\frac{1}{m_T} = -\frac{\sqrt{6}}{2}mN​=−mT​1​=−26​​.
  • The equation of the normal at MMM is: y−yM=mN(x−xM)y - y_M = m_N(x - x_M)y−yM​=mN​(x−xM​) y−6=−62(x−32)y - \sqrt{6} = -\frac{\sqrt{6}}{2} \left(x - \frac{3}{2}\right)y−6​=−26​​(x−23​)
  • To find where it meets the x-axis, set y=0y=0y=0: 0−6=−62(xQ−32)0 - \sqrt{6} = -\frac{\sqrt{6}}{2} \left(x_Q - \frac{3}{2}\right)0−6​=−26​​(xQ​−23​) 1=12(xQ−32)1 = \frac{1}{2} \left(x_Q - \frac{3}{2}\right)1=21​(xQ​−23​) 2=xQ−322 = x_Q - \frac{3}{2}2=xQ​−23​ xQ=2+32=72x_Q = 2 + \frac{3}{2} = \frac{7}{2}xQ​=2+23​=27​
  • Thus, the point QQQ is (7/2,0)(7/2, 0)(7/2,0).

6. Calculate Area of Triangle MQR

The vertices of the triangle are M(3/2,6)M(3/2, \sqrt{6})M(3/2,6​), Q(7/2,0)Q(7/2, 0)Q(7/2,0), and R(6,0)R(6, 0)R(6,0).

  • The base of the triangle lies on the x-axis, from QQQ to RRR. Length of base QR=∣xR−xQ∣=∣6−7/2∣=∣12/2−7/2∣=5/2QR = |x_R - x_Q| = |6 - 7/2| = |12/2 - 7/2| = 5/2QR=∣xR​−xQ​∣=∣6−7/2∣=∣12/2−7/2∣=5/2.
  • The height of the triangle is the y-coordinate of MMM, which is 6\sqrt{6}6​.
  • Area of △MQR=12×base×height=12×52×6=564\triangle MQR = \frac{1}{2} \times \text{base} \times \text{height} = \frac{1}{2} \times \frac{5}{2} \times \sqrt{6} = \frac{5\sqrt{6}}{4}△MQR=21​×base×height=21​×25​×6​=456​​.

7. Calculate Area of Quadrilateral MF1NF2

The vertices are M(3/2,6)M(3/2, \sqrt{6})M(3/2,6​), F1(−1,0)F_1(-1, 0)F1​(−1,0), N(3/2,−6)N(3/2, -\sqrt{6})N(3/2,−6​), and F2(1,0)F_2(1, 0)F2​(1,0).

  • This quadrilateral is a kite, with diagonals F1F2F_1F_2F1​F2​ and MNMNMN.
  • The length of diagonal F1F2F_1F_2F1​F2​ is ∣1−(−1)∣=2|1 - (-1)| = 2∣1−(−1)∣=2.
  • The length of diagonal MNMNMN is ∣6−(−6)∣=26|\sqrt{6} - (-\sqrt{6})| = 2\sqrt{6}∣6​−(−6​)∣=26​.
  • Area of a kite is 12d1d2\frac{1}{2} d_1 d_221​d1​d2​.
  • Area of quadrilateral MF1NF2=12×(2)×(26)=26M{F_1}N{F_2} = \frac{1}{2} \times (2) \times (2\sqrt{6}) = 2\sqrt{6}MF1​NF2​=21​×(2)×(26​)=26​.

8. Find the Ratio

The required ratio is: Area of △MQRArea of quadrilateral MF1NF2=56/426=564×126=58\frac{\text{Area of } \triangle MQR}{\text{Area of quadrilateral } M{F_1}N{F_2}} = \frac{5\sqrt{6}/4}{2\sqrt{6}} = \frac{5\sqrt{6}}{4} \times \frac{1}{2\sqrt{6}} = \frac{5}{8}Area of quadrilateral MF1​NF2​Area of △MQR​=26​56​/4​=456​​×26​1​=85​

  • The ratio is 5:85:85:8.

Comparing this with the options, the correct option is C.

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