Sign in
12thPass logo
New chatPYQ LibraryDoubtsRank report
Sign in to see Recents

Your guest activity stays on this device

Sign in to save progress →
Sign in

Parabola question

2011 · Shift 1 · Q32
Guest · filters and generic practice availableBrowsing as a guest · PYQ filters and generic practice are available. Sign in only for personalised features and saved progress.
  1. PYQ Library
  2. /JEE Advanced
  3. /Mathematics
  4. /Parabola
  5. /2011 · Shift 1 · Q32

Parabola question

2011 · Shift 1 · Q32

JEE AdvancedMathematicsParabolaNumerical+4 / −1
Consider the parabola y2=8x{y^2} = 8xy2=8x. Let Δ1{\Delta _1}Δ1​ be the area of the triangle formed by the end points of its latus rectum and the point P(12,2)P\left( {{1 \over 2},2} \right)P(21​,2) on the parabola and Δ2{\Delta _2}Δ2​ be the area of the triangle formed by drawing tangents at PPP and at the end points of the latus rectum. Then Δ1Δ2{{{\Delta _1}} \over {{\Delta _2}}}Δ2​Δ1​​ is
Numerical answer
View written solutionFree

Correct answer: 2

  1. Identify the parabola and its key points

    The parabola is y2=8xy^2=8xy2=8x Comparing with the standard form y2=4axy^2=4axy2=4ax we get 4a=8  ⟹  a=2.4a=8 \implies a=2.4a=8⟹a=2.

    So the parabola is y2=4axwith a=2.y^2=4ax \quad \text{with } a=2.y2=4axwith a=2.

  2. Find the endpoints of the latus rectum

    For the parabola y2=4axy^2=4axy2=4ax, the endpoints of the latus rectum are (a,2a)and(a,−2a).(a,2a) \quad \text{and} \quad (a,-2a).(a,2a)and(a,−2a).

    Since a=2a=2a=2, these are (2,4)and(2,−4).(2,4) \quad \text{and} \quad (2,-4).(2,4)and(2,−4).

  3. Given point PPP on the parabola

    The point is P(12,2).P\left(\frac12,2\right).P(21​,2).

    Check: y2=22=4,8x=8⋅12=4,y^2=2^2=4, \qquad 8x=8\cdot \frac12=4,y2=22=4,8x=8⋅21​=4, so PPP lies on the parabola.


Part A: Compute Δ1\Delta_1Δ1​

  1. Triangle formed by the latus rectum endpoints and PPP

    The three points are A=(2,4),B=(2,−4),P=(12,2).A=(2,4), \quad B=(2,-4), \quad P=\left(\frac12,2\right).A=(2,4),B=(2,−4),P=(21​,2).

    Segment ABABAB is vertical with length AB=4−(−4)=8.AB=4-(-4)=8.AB=4−(−4)=8.

    The perpendicular distance of PPP from the line x=2x=2x=2 is ∣2−12∣=32.\left|2-\frac12\right|=\frac32.​2−21​​=23​.

    Therefore, Δ1=12×8×32=6.\Delta_1=\frac12\times 8 \times \frac32=6.Δ1​=21​×8×23​=6.


Part B: Compute Δ2\Delta_2Δ2​

  1. Equation of tangent to y2=4axy^2=4axy2=4ax at point (x1,y1)(x_1,y_1)(x1​,y1​) on the parabola

    Standard tangent form is yy1=2a(x+x1).yy_1=2a(x+x_1).yy1​=2a(x+x1​).

    Here a=2a=2a=2, so yy1=4(x+x1).yy_1=4(x+x_1).yy1​=4(x+x1​).

  2. Tangents at the latus rectum endpoints

    • At (2,4)(2,4)(2,4): 4y=4(x+2)  ⟹  y=x+2.4y=4(x+2) \implies y=x+2.4y=4(x+2)⟹y=x+2.

    • At (2,−4)(2,-4)(2,−4): −4y=4(x+2)  ⟹  y=−x−2.-4y=4(x+2) \implies y=-x-2.−4y=4(x+2)⟹y=−x−2.

  3. Tangent at P(12,2)P\left(\frac12,2\right)P(21​,2)

    2y=4(x+12)2y=4\left(x+\frac12\right)2y=4(x+21​) 2y=4x+22y=4x+22y=4x+2 y=2x+1.y=2x+1.y=2x+1.

  4. Find the triangle formed by these three tangents

    The three lines are L1:y=x+2,L_1: y=x+2,L1​:y=x+2, L2:y=−x−2,L_2: y=-x-2,L2​:y=−x−2, L3:y=2x+1.L_3: y=2x+1.L3​:y=2x+1.

    Their pairwise intersections are:

    • L1L_1L1​ and L2L_2L2​: x+2=−x−2  ⟹  2x=−4  ⟹  x=−2,x+2=-x-2 \implies 2x=-4 \implies x=-2,x+2=−x−2⟹2x=−4⟹x=−2, y=0.y=0.y=0. So point is Q=(−2,0).Q=(-2,0).Q=(−2,0).

    • L1L_1L1​ and L3L_3L3​: x+2=2x+1  ⟹  x=1,x+2=2x+1 \implies x=1,x+2=2x+1⟹x=1, y=3.y=3.y=3. So point is R=(1,3).R=(1,3).R=(1,3).

    • L2L_2L2​ and L3L_3L3​: −x−2=2x+1  ⟹  −3x=3  ⟹  x=−1,-x-2=2x+1 \implies -3x=3 \implies x=-1,−x−2=2x+1⟹−3x=3⟹x=−1, y=−1.y=-1.y=−1. So point is S=(−1,−1).S=(-1,-1).S=(−1,−1).

  5. Area of triangle QRSQRSQRS

    Using the coordinate area formula: Δ2=12∣x1(y2−y3)+x2(y3−y1)+x3(y1−y2)∣.\Delta_2=\frac12 \left|x_1(y_2-y_3)+x_2(y_3-y_1)+x_3(y_1-y_2)\right|.Δ2​=21​∣x1​(y2​−y3​)+x2​(y3​−y1​)+x3​(y1​−y2​)∣.

    Substituting Q=(−2,0), R=(1,3), S=(−1,−1),Q=(-2,0),\ R=(1,3),\ S=(-1,-1),Q=(−2,0), R=(1,3), S=(−1,−1),

    Δ2=12[(−2)(3−(−1))+1((−1)−0)+(−1)(0−3)]abs\Delta_2=\frac12\left[(-2)(3-(-1))+1((-1)-0)+(-1)(0-3)\right]_{\text{abs}}Δ2​=21​[(−2)(3−(−1))+1((−1)−0)+(−1)(0−3)]abs​ =12[(−2)(4)+1(−1)+(−1)(−3)]abs=\frac12\left[(-2)(4)+1(-1)+(-1)(-3)\right]_{\text{abs}}=21​[(−2)(4)+1(−1)+(−1)(−3)]abs​ =12[−8−1+3]abs=\frac12\left[-8-1+3\right]_{\text{abs}}=21​[−8−1+3]abs​ =12∣−6∣=3.=\frac12\left|-6\right|=3.=21​∣−6∣=3.


Part C: Ratio

  1. Therefore, Δ1Δ2=63=2.\frac{\Delta_1}{\Delta_2}=\frac{6}{3}=2.Δ2​Δ1​​=36​=2.

So the required integer answer is 2\boxed{2}2​

The derived answer matches the stored correct answer.

PreviousNext

More from Parabola

  • Let (x,y) be any point on the parabola y2=4x. Let P be the point that divides the line segment from (0,0) to (x,y) in the ratio 1:3. Then the locus of P is2011 · MCQ
  • Let L be a normal to the parabola y2 = 4x. If L passes through the point (9, 6), then L is given by2011 · Multiple correct
  • Let A and B be two distinct points on the parabola y2=4x. If the axis of the parabola touches a circle of radius r having AB as its diameter, then the slope of the line joining A and B can be2010 · Multiple correct
  • The tangent PT and the normal PN to the parabola y2=4ax at a point P on it meet its axis at points T and N, respectively. The locus of the centroid of the triangle PTN is a parabola whose2009 · Multiple correct
  • The locus of the orthocentre of the triangle formed by the lines (1+p)x−py+p(1+p)=0,(1+q)x−qy+q(1+q)=0 and y=0, where peq, is :2009 · MCQ
  • The tangent to the curve y=ex drawn at the point (c,ec) intersects the line joining the points (c−1,ec−1) and (c+1,ec+1)2007 · MCQ
  • Consider the circle x2+y2=9 and the parabola y2=8x. They intersect at P and Q in the first and the fourth quadrants, respectively. Tangents to the circle at P and Q intersect the x-axis at R and tangents to the parabola…2007 · MCQ
  • Consider the circle x2+y2=9 and the parabola y2=8x. They intersect at P and Q in the first and the fourth quadrants, respectively. Tangents to the circle at P and Q intersect the x-axis at R and tangents to the parabola…2007 · MCQ