JEE Challenger
More from Conic Sections

Identification of Conic Sections from Loci and Parametric Forms

Match the conics in Column I with the statements/expressions in Column II.

Column IColumn II(A) Circle(p) The locus of the point (h,k) for which the linehx+ky=1 touches the circle x2+y2=4(B) Parabola(q) Points z in the complex plane satisfying∣z+2∣−∣z−2∣=±3(C) Ellipse(r) Points of the conic have parametricrepresentation x=3(1−t21+t2), y=2t1+t2(D) Hyperbola(s) The eccentricity of the conic lies in the interval1≤x<∞(t) Points z in the complex plane satisfyingRe(z+1)2=∣z∣2+1\begin{array}{ll} \textbf{Column I} & \textbf{Column II} \\ \text{(A) Circle} & \text{(p) The locus of the point } (h, k) \text{ for which the line} \\ & \quad hx + ky = 1 \text{ touches the circle } x^2 + y^2 = 4 \\ \text{(B) Parabola} & \text{(q) Points } z \text{ in the complex plane satisfying} \\ & \quad |z+2| - |z-2| = \pm 3 \\ \text{(C) Ellipse} & \text{(r) Points of the conic have parametric} \\ & \quad \text{representation } x = \sqrt{3}\left(\frac{1-t^2}{1+t^2}\right), \, y = \frac{2t}{1+t^2} \\ \text{(D) Hyperbola} & \text{(s) The eccentricity of the conic lies in the interval} \\ & \quad 1 \le x < \infty \\ & \text{(t) Points } z \text{ in the complex plane satisfying} \\ & \quad \text{Re}(z+1)^2 = |z|^2 + 1 \end{array}

Options

A

(A) →\to (p)

Correct
B

(B) →\to (s), (t)

Correct
C

(C) →\to (r)

Correct
D

(D) →\to (q), (s)

Correct

Step-by-Step Solution

To determine the correct matches between Column I and Column II, we analyze each statement in Column II step-by-step:

1. Statement (p)

The line is given by hx+ky=1hx + ky = 1 and it touches the circle x2+y2=4x^2 + y^2 = 4 (radius r=2r = 2, center (0,0)(0,0)).
The perpendicular distance from the center (0,0)(0,0) to the line is equal to the radius:
∣h(0)+k(0)−1∣h2+k2=2  ⟹  1h2+k2=2  ⟹  h2+k2=14\frac{|h(0) + k(0) - 1|}{\sqrt{h^2 + k^2}} = 2 \implies \frac{1}{\sqrt{h^2 + k^2}} = 2 \implies h^2 + k^2 = \frac{1}{4}
Replacing (h,k)(h, k) with (x,y)(x, y), the locus is:
x2+y2=14x^2 + y^2 = \frac{1}{4}
This is the equation of a Circle.
Hence, (A) →\to (p).


2. Statement (q)

The given condition in the complex plane is:
∣z+2∣−∣z−2∣=±3|z + 2| - |z - 2| = \pm 3
Let the two fixed points (foci) be S1(−2,0)S_1(-2, 0) and S2(2,0)S_2(2, 0).
The distance between the foci is 2ae=42ae = 4.
The locus represents points P(z)P(z) such that the absolute difference between their distances to the two fixed points is constant:
∣∣z−(−2)∣−∣z−2∣∣=3||z - (-2)| - |z - 2|| = 3
Since 2a=3<4=2ae2a = 3 < 4 = 2ae, we have the eccentricity e=43>1e = \frac{4}{3} > 1.
This defines a Hyperbola.
Hence, (D) →\to (q).


3. Statement (r)

The parametric equations are:
x=3(1−t21+t2),y=2t1+t2x = \sqrt{3}\left(\frac{1-t^2}{1+t^2}\right), \quad y = \frac{2t}{1+t^2}
Substituting t=tan⁡(θ2)t = \tan\left(\frac{\theta}{2}\right), we use the standard half-angle identities cos⁡θ=1−t21+t2\cos\theta = \frac{1-t^2}{1+t^2} and sin⁡θ=2t1+t2\sin\theta = \frac{2t}{1+t^2} to get:
x=3cos⁡θ,y=sin⁡θx = \sqrt{3}\cos\theta, \quad y = \sin\theta
Eliminating the parameter θ\theta:
x23+y21=cos⁡2θ+sin⁡2θ=1\frac{x^2}{3} + \frac{y^2}{1} = \cos^2\theta + \sin^2\theta = 1
This is the standard equation of an Ellipse.
Hence, (C) →\to (r).


4. Statement (s)

The eccentricity ee of the conic lies in the interval:
1≤e<∞1 \le e < \infty

  • For a Parabola, the eccentricity is e=1∈[1,∞)e = 1 \in [1, \infty).
  • For a Hyperbola, the eccentricity is e>1e > 1, which also lies in [1,∞)[1, \infty).
    Hence, (B) →\to (s) and (D) →\to (s).

5. Statement (t)

Let z=x+iyz = x + iy. Then:
z+1=(x+1)+iyz + 1 = (x + 1) + iy
(z+1)2=((x+1)+iy)2=(x+1)2−y2+2i(x+1)y(z + 1)^2 = ((x + 1) + iy)^2 = (x + 1)^2 - y^2 + 2i(x + 1)y
Taking the real part:
Re(z+1)2=(x+1)2−y2=x2+2x+1−y2\text{Re}(z + 1)^2 = (x + 1)^2 - y^2 = x^2 + 2x + 1 - y^2
The right-hand side is:
∣z∣2+1=(x2+y2)+1|z|^2 + 1 = (x^2 + y^2) + 1
Equating both sides:
x2+2x+1−y2=x2+y2+1  ⟹  2x=2y2  ⟹  y2=xx^2 + 2x + 1 - y^2 = x^2 + y^2 + 1 \implies 2x = 2y^2 \implies y^2 = x
This is the equation of a Parabola.
Hence, (B) →\to (t).


Conclusion:

  • (A) →\to (p)
  • (B) →\to (s), (t)
  • (C) →\to (r)
  • (D) →\to (q), (s)

All the given options A, B, C, and D are correct.