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Parabola question

2017 · Shift 1 · Q33
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Parabola question

2017 · Shift 1 · Q33

JEE AdvancedMathematicsParabolaMCQ+3 / −1
By appropriately matching the information given in the three columns of the following table.

Columns 1, 2 and 3 contain conics, equations of tangents to the conics and points of contact, respectively.

Column - 1 Column - 2 Column - 3
(i) x2+y2=a{x^2} + {y^2} = ax2+y2=a my=m2x+amy = {m^2}x + amy=m2x+a (am2, 2am)\left( {{a \over {{m^2}}},\,{{2a} \over m}} \right)(m2a​,m2a​)
(ii) x2a2y2=a2]{x^2}{a^2}{y^2} = {a^2}]x2a2y2=a2] y=mx+am2+1y = mx + a\sqrt {{m^2} + 1}y=mx+am2+1​ (−mam2+1, am2+1)\left( {{{ - ma} \over {\sqrt {{m^2} + 1} }},\,{a \over {\sqrt {{m^2} + 1} }}} \right)(m2+1​−ma​,m2+1​a​)
(iii) y2=4ax{y^2} = 4axy2=4ax y=mx+a2m2−1y = mx + \sqrt {{a^2}{m^2} - 1}y=mx+a2m2−1​ (−a2ma2m2+1, 1a2m2+1)\left( {{{ - {a^2}m} \over {\sqrt {{a^2}{m^2} + 1} }},\,{1 \over {\sqrt {{a^2}{m^2} + 1} }}} \right)(a2m2+1​−a2m​,a2m2+1​1​)
(iv) x2−a2y2=a2{x^2} - {a^2}{y^2} = {a^2}x2−a2y2=a2 y=mx+a2m2+1y = mx + \sqrt {{a^2}{m^2} + 1}y=mx+a2m2+1​ (−a2ma2m2−1, −1a2m2−1)\left( {{{ - {a^2}m} \over {\sqrt {{a^2}{m^2} - 1} }},\,{{ - 1} \over {\sqrt {{a^2}{m^2} - 1} }}} \right)(a2m2−1​−a2m​,a2m2−1​−1​)
If a tangent to a suitable conic (Column 1) is found to be y = x + 8 and its point of contact is (8, 16), then which of the following options is the only CORRECT combination?
  1. A
    (III) (i) (P)
  2. B
    (I) (ii) (Q)
  3. C
    (II) (iv) (R)
  4. D
    (III) (ii) (Q)
View written solutionFree

Correct answer: A

Step 1: Analyze and Match the Columns

First, we need to correctly match the conic in Column 1 with its corresponding tangent equation in Column 2 and point of contact in Column 3. We will assume standard forms for the conics, correcting for apparent typos in the question.

  • Conic (I): Circle x2+y2=a2x^2 + y^2 = a^2x2+y2=a2 (assuming the typo aaa should be a2a^2a2).

    • The standard equation of a tangent with slope mmm to this circle is y=mx±am2+1y = mx \pm a\sqrt{m^2 + 1}y=mx±am2+1​. Column 2, option (ii) is y=mx+am2+1y = mx + a\sqrt{m^2 + 1}y=mx+am2+1​.
    • The point of contact for this tangent is (−ma2±am2+1,a2±am2+1)=(∓mam2+1,±am2+1)\left( \frac{-ma^2}{\pm a\sqrt{m^2+1}}, \frac{a^2}{\pm a\sqrt{m^2+1}} \right) = \left( \mp \frac{ma}{\sqrt{m^2+1}}, \pm \frac{a}{\sqrt{m^2+1}} \right)(±am2+1​−ma2​,±am2+1​a2​)=(∓m2+1​ma​,±m2+1​a​). For the tangent with the '+' sign, the point is (−mam2+1,am2+1)\left( \frac{-ma}{\sqrt{m^2+1}}, \frac{a}{\sqrt{m^2+1}} \right)(m2+1​−ma​,m2+1​a​). This matches Column 3, option (Q).
    • So, the correct combination for the circle is (I) →\rightarrow→ (ii) →\rightarrow→ (Q).
  • Conic (III): Parabola y2=4axy^2 = 4axy2=4ax.

    • The standard equation of a tangent with slope mmm is y=mx+amy = mx + \frac{a}{m}y=mx+ma​. This can be rewritten as my=m2x+amy = m^2x + amy=m2x+a, which matches Column 2, option (i).
    • The point of contact for this tangent is (am2,2am)\left( \frac{a}{m^2}, \frac{2a}{m} \right)(m2a​,m2a​). This matches Column 3, option (P).
    • So, the correct combination for the parabola is (III) →\rightarrow→ (i) →\rightarrow→ (P).
  • Conic (II): Ellipse x2a2+y2=1\frac{x^2}{a^2} + y^2 = 1a2x2​+y2=1 (assuming from x2+a2y2=a2x^2 + a^2y^2 = a^2x2+a2y2=a2).

    • The standard equation of a tangent with slope mmm is y=mx±a2m2+1y = mx \pm \sqrt{a^2m^2 + 1}y=mx±a2m2+1​. Column 2, option (iv) is y=mx+a2m2+1y = mx + \sqrt{a^2m^2 + 1}y=mx+a2m2+1​.
    • The point of contact for this tangent is (−a2m±a2m2+1,1±a2m2+1)\left( \frac{-a^2m}{\pm\sqrt{a^2m^2+1}}, \frac{1}{\pm\sqrt{a^2m^2+1}} \right)(±a2m2+1​−a2m​,±a2m2+1​1​). For the tangent with the '+' sign, the point is (−a2ma2m2+1,1a2m2+1)\left( \frac{-a^2m}{\sqrt{a^2m^2+1}}, \frac{1}{\sqrt{a^2m^2+1}} \right)(a2m2+1​−a2m​,a2m2+1​1​). This matches Column 3, option (R).
    • So, the correct combination for the ellipse is (II) →\rightarrow→ (iv) →\rightarrow→ (R).
  • Conic (IV): Hyperbola x2a2−y2=1\frac{x^2}{a^2} - y^2 = 1a2x2​−y2=1 (assuming from x2−a2y2=a2x^2 - a^2y^2 = a^2x2−a2y2=a2).

    • The standard equation of a tangent with slope mmm is y=mx±a2m2−1y = mx \pm \sqrt{a^2m^2 - 1}y=mx±a2m2−1​. Column 2, option (iii) is y=mx+a2m2−1y = mx + \sqrt{a^2m^2 - 1}y=mx+a2m2−1​.
    • The point of contact is (−a2m±a2m2−1,−1±a2m2−1)\left( \frac{-a^2m}{\pm\sqrt{a^2m^2-1}}, \frac{-1}{\pm\sqrt{a^2m^2-1}} \right)(±a2m2−1​−a2m​,±a2m2−1​−1​). For the tangent with the '+' sign, the point is (−a2ma2m2−1,−1a2m2−1)\left( \frac{-a^2m}{\sqrt{a^2m^2-1}}, \frac{-1}{\sqrt{a^2m^2-1}} \right)(a2m2−1​−a2m​,a2m2−1​−1​). This matches Column 3, option (S).
    • So, the correct combination for the hyperbola is (IV) →\rightarrow→ (iii) →\rightarrow→ (S).

Step 2: Use the Given Specific Information

We are given that a tangent to one of these conics is the line y=x+8y = x + 8y=x+8 and its point of contact is (8,16)(8, 16)(8,16).

First, let's verify that the point (8,16)(8, 16)(8,16) lies on the line y=x+8y = x + 8y=x+8. Substituting x=8x=8x=8 gives y=8+8=16y = 8 + 8 = 16y=8+8=16. The point lies on the line, so the information is consistent.

From the tangent equation y=x+8y = x + 8y=x+8, we can identify the slope m=1m=1m=1 and the y-intercept c=8c=8c=8.

Step 3: Test each Conic Combination

We will now check which of the matched combinations from Step 1 fits this specific data.

  1. Parabola (III) →\rightarrow→ (i) →\rightarrow→ (P):

    • Conic: y2=4axy^2 = 4axy2=4ax.
    • Tangent: y=mx+amy = mx + \frac{a}{m}y=mx+ma​.
    • Point of contact: (am2,2am)\left( \frac{a}{m^2}, \frac{2a}{m} \right)(m2a​,m2a​).
    • From the given tangent, m=1m=1m=1. The equation becomes y=x+ay = x + ay=x+a. Comparing this with y=x+8y = x + 8y=x+8, we get a=8a=8a=8.
    • Now, let's find the point of contact using a=8a=8a=8 and m=1m=1m=1: (812,2(8)1)=(8,16)\left( \frac{8}{1^2}, \frac{2(8)}{1} \right) = (8, 16)(128​,12(8)​)=(8,16)This matches the given point of contact.
    • The conic is y2=4(8)x=32xy^2 = 4(8)x = 32xy2=4(8)x=32x. Let's check if the point (8,16)(8, 16)(8,16) lies on it: 162=25616^2 = 256162=256 and 32×8=25632 \times 8 = 25632×8=256. The point is on the conic.
    • This combination is a perfect match for the given information.
  2. Circle (I) →\rightarrow→ (ii) →\rightarrow→ (Q):

    • Tangent intercept c=am2+1c = a\sqrt{m^2+1}c=am2+1​. With m=1m=1m=1 and c=8c=8c=8, we have 8=a12+1=a28 = a\sqrt{1^2+1} = a\sqrt{2}8=a12+1​=a2​, so a=42a = 4\sqrt{2}a=42​.
    • Point of contact: (−mam2+1,am2+1)=(−1(42)2,422)=(−4,4)\left( \frac{-ma}{\sqrt{m^2+1}}, \frac{a}{\sqrt{m^2+1}} \right) = \left( \frac{-1(4\sqrt{2})}{\sqrt{2}}, \frac{4\sqrt{2}}{\sqrt{2}} \right) = (-4, 4)(m2+1​−ma​,m2+1​a​)=(2​−1(42​)​,2​42​​)=(−4,4).
    • This does not match (8,16)(8, 16)(8,16).
  3. Ellipse (II) →\rightarrow→ (iv) →\rightarrow→ (R):

    • Tangent intercept c=a2m2+1c = \sqrt{a^2m^2+1}c=a2m2+1​. With m=1m=1m=1 and c=8c=8c=8, we have 8=a2(1)2+18 = \sqrt{a^2(1)^2+1}8=a2(1)2+1​, so 64=a2+164 = a^2+164=a2+1, which gives a2=63a^2=63a2=63.
    • Point of contact: (−a2ma2m2+1,1a2m2+1)=(−63(1)8,18)\left( \frac{-a^2m}{\sqrt{a^2m^2+1}}, \frac{1}{\sqrt{a^2m^2+1}} \right) = \left( \frac{-63(1)}{8}, \frac{1}{8} \right)(a2m2+1​−a2m​,a2m2+1​1​)=(8−63(1)​,81​).
    • This does not match (8,16)(8, 16)(8,16).
  4. Hyperbola (IV) →\rightarrow→ (iii) →\rightarrow→ (S):

    • Tangent intercept c=a2m2−1c = \sqrt{a^2m^2-1}c=a2m2−1​. With m=1m=1m=1 and c=8c=8c=8, we have 8=a2(1)2−18 = \sqrt{a^2(1)^2-1}8=a2(1)2−1​, so 64=a2−164 = a^2-164=a2−1, which gives a2=65a^2=65a2=65.
    • Point of contact: (−a2ma2m2−1,−1a2m2−1)=(−65(1)8,−18)\left( \frac{-a^2m}{\sqrt{a^2m^2-1}}, \frac{-1}{\sqrt{a^2m^2-1}} \right) = \left( \frac{-65(1)}{8}, \frac{-1}{8} \right)(a2m2−1​−a2m​,a2m2−1​−1​)=(8−65(1)​,8−1​).
    • This does not match (8,16)(8, 16)(8,16).

Step 4: Conclusion

The only conic that fits the given tangent line and point of contact is the parabola, corresponding to the combination (III) →\rightarrow→ (i) →\rightarrow→ (P). Therefore, the only correct combination among the options is (III) (i) (P).

Looking at the options: A: (III) (i) (P) B: (I) (ii) (Q) C: (II) (iv) (R) D: (III) (ii) (Q)

Option A is the correct choice.

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