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Parabola question

2022 · 26 Jul · Shift 2 · Q23
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  5. /2022 · 26 Jul · Shift 2 · Q23

Parabola question

2022 · 26 Jul · Shift 2 · Q23

JEE MainMathematicsParabolaMCQ+4 / −1
Let P\mathrm{P}P and Q\mathrm{Q}Q be any points on the curves (x−1)2+(y+1)2=1(x-1)^{2}+(y+1)^{2}=1(x−1)2+(y+1)2=1 and y=x2y=x^{2}y=x2, respectively. The distance between PPP and QQQ is minimum for some value of the abscissa of PPP in the interval :
  1. A
    (0,14)\left(0, \frac{1}{4}\right)(0,41​)
  2. B
    (12,34)\left(\frac{1}{2}, \frac{3}{4}\right)(21​,43​)
  3. C
    (14,12)\left(\frac{1}{4}, \frac{1}{2}\right)(41​,21​)
  4. D
    (34,1)\left(\frac{3}{4}, 1\right)(43​,1)
View written solutionFree

Correct answer: C

  1. Interpret the curves

    • Point PPP lies on the circle (x−1)2+(y+1)2=1(x-1)^2+(y+1)^2=1(x−1)2+(y+1)2=1 which has center C=(1,−1)C=(1,-1)C=(1,−1) and radius 111.

    • Point QQQ lies on the parabola y=x2.y=x^2.y=x2.

    We need the point PPP on the circle for which the distance to the parabola is minimum.


  1. Geometric idea for minimum distance between two smooth curves

    If the minimum distance occurs at points PPP and QQQ on the two curves, then the segment PQPQPQ is along the common normal to both curves.

    Also, for a circle, the normal at PPP is along the radius CPCPCP. Hence at the minimizing position, the line joining PPP to QQQ must pass through the center C=(1,−1)C=(1,-1)C=(1,−1).

    Therefore, if Q=(t,t2)Q=(t,t^2)Q=(t,t2) is the corresponding point on the parabola, then the normal to the parabola at QQQ must pass through (1,−1)(1,-1)(1,−1).


  1. Equation of normal to the parabola y=x2y=x^2y=x2

    Let Q=(t,t2)Q=(t,t^2)Q=(t,t2) on y=x2y=x^2y=x2.

    Slope of tangent at QQQ is dydx=2t.\frac{dy}{dx}=2t.dxdy​=2t.

    So slope of normal is −12t(t≠0).-\frac{1}{2t} \quad (t\neq 0).−2t1​(t=0).

    Equation of the normal at QQQ is y−t2=−12t(x−t).y-t^2=-\frac{1}{2t}(x-t).y−t2=−2t1​(x−t).

    Since this normal passes through (1,−1)(1,-1)(1,−1), substitute x=1x=1x=1, y=−1y=-1y=−1: −1−t2=−12t(1−t).-1-t^2=-\frac{1}{2t}(1-t).−1−t2=−2t1​(1−t).

    Multiply by 2t2t2t: −2t−2t3=−(1−t).-2t-2t^3=-(1-t).−2t−2t3=−(1−t).

    Hence, 2t+2t3=1−t2t+2t^3=1-t2t+2t3=1−t 2t3+3t−1=0.2t^3+3t-1=0.2t3+3t−1=0.


  1. Solve the cubic

    We test t=12t=\frac12t=21​:

    =\frac14+\frac32-1 =\frac34\neq 0. $$ Test $t=\frac13$: $$ 2\left(\frac{1}{27}\right)+3\left(\frac13\right)-1 =\frac{2}{27}+1-1 =\frac{2}{27}\neq 0. $$ Test $t\approx 0.3$: $$ 2(0.027)+0.9-1=-0.046. $$ Test $t\approx 0.32$: $$ 2(0.032768)+0.96-1=0.025536. $$ So the root lies near $t\approx 0.31$. We now find the corresponding point $P$ on the circle.

  1. Find PPP from the center toward QQQ

    Since PPP is on the circle and lies on the line from center C=(1,−1)C=(1,-1)C=(1,−1) toward Q=(t,t2)Q=(t,t^2)Q=(t,t2), we have P=C+Q−C∣Q−C∣.P=C+\frac{Q-C}{|Q-C|}.P=C+∣Q−C∣Q−C​.

    Here Q−C=(t−1,t2+1).Q-C=(t-1,t^2+1).Q−C=(t−1,t2+1).

    The abscissa of PPP is xP=1+t−1(t−1)2+(t2+1)2.x_P=1+\frac{t-1}{\sqrt{(t-1)^2+(t^2+1)^2}}.xP​=1+(t−1)2+(t2+1)2​t−1​.

    Using t≈0.31t\approx 0.31t≈0.31: t−1≈−0.69,t2+1≈1.0961.t-1\approx -0.69, \qquad t^2+1\approx 1.0961.t−1≈−0.69,t2+1≈1.0961.

    Then

    \approx \sqrt{0.4761+1.2014} \approx \sqrt{1.6775} \approx 1.295. $$ Therefore, $$ x_P\approx 1-\frac{0.69}{1.295} \approx 1-0.533 \approx 0.467. $$ Thus the abscissa of $P$ lies in $$ \left(\frac14,\frac12\right). $$

  1. Check options

    • A: (0,14)\left(0,\frac14\right)(0,41​) ❌
    • B: (12,34)\left(\frac12,\frac34\right)(21​,43​) ❌
    • C: (14,12)\left(\frac14,\frac12\right)(41​,21​) ✅
    • D: (34,1)\left(\frac34,1\right)(43​,1) ❌

So the correct option is C.


  1. Comparison with stored answer

    Stored correct answer: C

    Our derived answer: C

    Hence, they agree.

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