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Application of Derivatives question

2025 · 3 Apr · Shift 2 · Q31
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Application of Derivatives question

2025 · 3 Apr · Shift 2 · Q31

JEE MainMathematicsApplication of DerivativesMCQ+4 / −1
The shortest distance between the curves y2=8xy^2=8 xy2=8x and x2+y2+12y+35=0x^2+y^2+12 y+35=0x2+y2+12y+35=0 is:
  1. A
    23−12 \sqrt{3}-123​−1
  2. B
    22−12 \sqrt{2}-122​−1
  3. C
    32−13 \sqrt{2}-132​−1
  4. D
    2\sqrt{2}2​
View written solutionFree

Correct answer: B

  1. Identify the curves

    The two curves are:

    • Parabola: y2=8xy^2=8xy2=8x
    • Circle: x2+y2+12y+35=0x^2+y^2+12y+35=0x2+y2+12y+35=0
  2. Rewrite the circle in standard form

    Complete the square in yyy: x2+y2+12y+35=0x^2+y^2+12y+35=0x2+y2+12y+35=0 x2+(y+6)2−36+35=0x^2+(y+6)^2-36+35=0x2+(y+6)2−36+35=0 x2+(y+6)2=1x^2+(y+6)^2=1x2+(y+6)2=1

    So this is a circle with:

    • Centre C=(0,−6)C=(0,-6)C=(0,−6)
    • Radius r=1r=1r=1
  3. Reduce the problem

    The shortest distance between the parabola and the circle equals:

    (shortest distance from centre of circle to parabola)−r\text{(shortest distance from centre of circle to parabola)} - r(shortest distance from centre of circle to parabola)−r

    because the nearest point on the circle to an external point lies along the radius.

    So we first find the minimum distance from (0,−6)(0,-6)(0,−6) to the parabola y2=8xy^2=8xy2=8x.

  4. Take a general point on the parabola

    Since y2=8x=4axy^2=8x=4axy2=8x=4ax with a=2a=2a=2, a standard parametric point on the parabola is P=(2t2,4t)P=(2t^2,4t)P=(2t2,4t)

  5. Distance from (0,−6)(0,-6)(0,−6) to PPP

    Let the centre be C=(0,−6)C=(0,-6)C=(0,−6). Then CP2=(2t2−0)2+(4t+6)2CP^2=(2t^2-0)^2+(4t+6)^2CP2=(2t2−0)2+(4t+6)2 CP2=4t4+16t2+48t+36CP^2=4t^4+16t^2+48t+36CP2=4t4+16t2+48t+36

    We minimize f(t)=4t4+16t2+48t+36f(t)=4t^4+16t^2+48t+36f(t)=4t4+16t2+48t+36.

  6. Differentiate and find critical points

    f′(t)=16t3+32t+48f'(t)=16t^3+32t+48f′(t)=16t3+32t+48 f′(t)=16(t3+2t+3)f'(t)=16(t^3+2t+3)f′(t)=16(t3+2t+3)

    Set this equal to zero: t3+2t+3=0t^3+2t+3=0t3+2t+3=0

    Try t=−1t=-1t=−1: (−1)3+2(−1)+3=−1−2+3=0(-1)^3+2(-1)+3=-1-2+3=0(−1)3+2(−1)+3=−1−2+3=0

    Hence, t3+2t+3=(t+1)(t2−t+3)t^3+2t+3=(t+1)(t^2-t+3)t3+2t+3=(t+1)(t2−t+3)

    Since t2−t+3>0t^2-t+3>0t2−t+3>0 for all real ttt, the only real critical point is t=−1t=-1t=−1

  7. Check minimum distance

    Since the quartic has positive leading coefficient and only one real critical point, this gives the minimum.

    At t=−1t=-1t=−1, P=(2,−4)P=(2, -4)P=(2,−4)

    Then CP=(2−0)2+(−4+6)2=4+4=22CP=\sqrt{(2-0)^2+(-4+6)^2}=\sqrt{4+4}=2\sqrt{2}CP=(2−0)2+(−4+6)2​=4+4​=22​

  8. Distance from circle to parabola

    The circle has radius 111, so the shortest distance between the circle and parabola is 22−12\sqrt{2}-122​−1

  9. Match with options

    22−12\sqrt{2}-122​−1 corresponds to Option B.

  10. Comparison with stored answer

Stored correct answer: B

Our derived answer: B

Hence they agree.

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