JEE Challenger
More from Complex Numbers and Quadratic Equations

Match Complex Numbers Functions Determinants and Derivatives to Intervals

Match the statements given in Column I with the intervals/union of intervals given in Column II

\begin{array}{l&l} \text{\textbf{Column I}} & \text{\textbf{Column II}} \\ \text{(A) The set } \left\{\text{Re}\left(\frac{2iz}{1-z^2}\right) : z \text{ is a complex number, } |z| = 1, z \neq \pm 1\right\} \text{ is} & \text{(p) } (-\infty, -1) \cup (1, \infty) \\ \text{(B) The domain of the function } f(x) = \sin^{-1}\left(\frac{8(3)^{x-2}}{1-3^{2(x-1)}}\right) \text{ is} & \text{(q) } (-\infty, 0) \cup (0, \infty) \\ \text{(C) If } f(\theta) = \begin{vmatrix} 1 & \tan\theta & 1 \\ -\tan\theta & 1 & \tan\theta \\ -1 & -\tan\theta & 1 \end{vmatrix}, \text{ then the set } \left\{f(\theta) : 0 \le \theta < \frac{\pi}{2}\right\} \text{ is} & \text{(r) } [2, \infty) \\ \text{(D) If } f(x) = x^{3/2}(3x-10), \quad x \ge 0, \text{ then } f(x) \text{ is increasing in} & \text{(s) } (-\infty, -1] \cup [1, \infty) \\ & \text{(t) } (-\infty, 0] \cup [2, \infty) \end{array}

Options

A

(A)→(S),  (B)→(R),  (C)→(T),  (D)→(R)(\text{A}) \rightarrow (\text{S}),\; (\text{B}) \rightarrow (\text{R}),\; (\text{C}) \rightarrow (\text{T}),\; (\text{D}) \rightarrow (\text{R})

B

(A)→(T),  (B)→(S),  (C)→(R),  (D)→(R)(\text{A}) \rightarrow (\text{T}),\; (\text{B}) \rightarrow (\text{S}),\; (\text{C}) \rightarrow (\text{R}),\; (\text{D}) \rightarrow (\text{R})

C

(A)→(S),  (B)→(T),  (C)→(P),  (D)→(R)(\text{A}) \rightarrow (\text{S}),\; (\text{B}) \rightarrow (\text{T}),\; (\text{C}) \rightarrow (\text{P}),\; (\text{D}) \rightarrow (\text{R})

D

(A)→(S),  (B)→(T),  (C)→(R),  (D)→(R)(\text{A}) \rightarrow (\text{S}),\; (\text{B}) \rightarrow (\text{T}),\; (\text{C}) \rightarrow (\text{R}),\; (\text{D}) \rightarrow (\text{R})

Correct

Step-by-Step Solution

To match the statements given in Column I with the correct intervals/union of intervals in Column II, we evaluate each part step-by-step:


Part (A)

We are given the set:
{Re(2iz1−z2):z is a complex number, ∣z∣=1,z≠±1}\left\{\text{Re}\left(\frac{2iz}{1-z^2}\right) : z \text{ is a complex number, } |z| = 1, z \neq \pm 1\right\}

Since ∣z∣=1|z| = 1 and z≠±1z \neq \pm 1, we can express zz in polar form as z=eiθ=cos⁡θ+isin⁡θz = e^{i\theta} = \cos\theta + i\sin\theta, where θ∈R∖{kπ:k∈Z}\theta \in \mathbb{R} \setminus \{k\pi : k \in \mathbb{Z}\}.

Substituting z=eiθz = e^{i\theta} into the given expression:
w=2iz1−z2=2ieiθ1−e2iθw = \frac{2iz}{1-z^2} = \frac{2i e^{i\theta}}{1 - e^{2i\theta}}

Dividing the numerator and denominator by eiθe^{i\theta}:
w=2ie−iθ−eiθ=2i−2isin⁡θ=−1sin⁡θw = \frac{2i}{e^{-i\theta} - e^{i\theta}} = \frac{2i}{-2i\sin\theta} = -\frac{1}{\sin\theta}

Since θ≠kπ\theta \neq k\pi, the range of sin⁡θ\sin\theta is [−1,0)∪(0,1][-1, 0) \cup (0, 1].
Consequently, the value of ww lies in:
w∈(−∞,−1]∪[1,∞)w \in (-\infty, -1] \cup [1, \infty)

Since ww is purely real, Re(w)=w∈(−∞,−1]∪[1,∞)\text{Re}(w) = w \in (-\infty, -1] \cup [1, \infty).

Thus, (A) matches with (s).


Part (B)

We are given the function:
f(x)=sin⁡−1(8(3)x−21−32(x−1))f(x) = \sin^{-1}\left(\frac{8(3)^{x-2}}{1 - 3^{2(x-1)}}\right)

For f(x)f(x) to be defined, the argument of sin⁡−1\sin^{-1} must satisfy:
−1≤8⋅3x−21−32(x−1)≤1  ⟹  ∣8⋅3x−21−32(x−1)∣≤1-1 \le \frac{8 \cdot 3^{x-2}}{1 - 3^{2(x-1)}} \le 1 \implies \left| \frac{8 \cdot 3^{x-2}}{1 - 3^{2(x-1)}} \right| \le 1

Let t=3x−1>0t = 3^{x-1} > 0. Then 3x−2=t33^{x-2} = \frac{t}{3} and 32(x−1)=t23^{2(x-1)} = t^2. Substituting these yields:
∣8t31−t2∣≤1  ⟹  8t≤3∣1−t2∣(t≠1)\left| \frac{\frac{8t}{3}}{1 - t^2} \right| \le 1 \implies 8t \le 3|1 - t^2| \quad (t \neq 1)

We consider two cases based on the value of tt:

  1. Case 1: 0<t<10 < t < 1 (i.e., 1−t2>01 - t^2 > 0)
    8t≤3(1−t2)  ⟹  3t2+8t−3≤0  ⟹  (3t−1)(t+3)≤08t \le 3(1 - t^2) \implies 3t^2 + 8t - 3 \le 0 \implies (3t - 1)(t + 3) \le 0
    Since t>0t > 0, this inequality simplifies to:
    0<t≤13  ⟹  3x−1≤3−1  ⟹  x−1≤−1  ⟹  x≤00 < t \le \frac{1}{3} \implies 3^{x-1} \le 3^{-1} \implies x - 1 \le -1 \implies x \le 0
    So, x∈(−∞,0]x \in (-\infty, 0].

  2. Case 2: t>1t > 1 (i.e., 1−t2<01 - t^2 < 0)
    8t≤3(t2−1)  ⟹  3t2−8t−3≥0  ⟹  (3t+1)(t−3)≥08t \le 3(t^2 - 1) \implies 3t^2 - 8t - 3 \ge 0 \implies (3t + 1)(t - 3) \ge 0
    Since t>1t > 1, this inequality simplifies to:
    t≥3  ⟹  3x−1≥31  ⟹  x−1≥1  ⟹  x≥2t \ge 3 \implies 3^{x-1} \ge 3^1 \implies x - 1 \ge 1 \implies x \ge 2
    So, x∈[2,∞)x \in [2, \infty).

Combining both cases, the domain of f(x)f(x) is (−∞,0]∪[2,∞)(-\infty, 0] \cup [2, \infty).

Thus, (B) matches with (t).


Part (C)

We are given the determinant:

f(θ)=∣1tan⁡θ1−tan⁡θ1tan⁡θ−1−tan⁡θ1∣f(\theta) = \begin{vmatrix} 1 & \tan\theta & 1 \\ -\tan\theta & 1 & \tan\theta \\ -1 & -\tan\theta & 1 \end{vmatrix}

Expanding the determinant along the first row:
f(θ)=1⋅(1+tan⁡2θ)−tan⁡θ⋅(−tan⁡θ+tan⁡θ)+1⋅(tan⁡2θ+1)f(\theta) = 1 \cdot (1 + \tan^2\theta) - \tan\theta \cdot (-\tan\theta + \tan\theta) + 1 \cdot (\tan^2\theta + 1)
f(θ)=(1+tan⁡2θ)−0+(1+tan⁡2θ)=2(1+tan⁡2θ)=2sec⁡2θf(\theta) = (1 + \tan^2\theta) - 0 + (1 + \tan^2\theta) = 2(1 + \tan^2\theta) = 2\sec^2\theta

For 0≤θ<π20 \le \theta < \frac{\pi}{2}, we have sec⁡θ≥1  ⟹  sec⁡2θ∈[1,∞)\sec\theta \ge 1 \implies \sec^2\theta \in [1, \infty).
Therefore:
f(θ)=2sec⁡2θ∈[2,∞)f(\theta) = 2\sec^2\theta \in [2, \infty)

Thus, (C) matches with (r).


Part (D)

We are given the function:
f(x)=x3/2(3x−10)=3x5/2−10x3/2,x≥0f(x) = x^{3/2}(3x - 10) = 3x^{5/2} - 10x^{3/2}, \quad x \ge 0

Differentiating f(x)f(x) with respect to xx:
f′(x)=3⋅52x3/2−10⋅32x1/2=152x3/2−15x1/2=152x(x−2)f'(x) = 3 \cdot \frac{5}{2}x^{3/2} - 10 \cdot \frac{3}{2}x^{1/2} = \frac{15}{2}x^{3/2} - 15x^{1/2} = \frac{15}{2}\sqrt{x}(x - 2)

For f(x)f(x) to be increasing, we require f′(x)≥0f'(x) \ge 0:
152x(x−2)≥0\frac{15}{2}\sqrt{x}(x - 2) \ge 0

Since x≥0\sqrt{x} \ge 0 for all x≥0x \ge 0, f′(x)≥0f'(x) \ge 0 holds when:
x−2≥0  ⟹  x≥2x - 2 \ge 0 \implies x \ge 2

Thus, f(x)f(x) is increasing in [2,∞)[2, \infty).

Thus, (D) matches with (r).


Final Match:

  • (A) →\rightarrow (s)
  • (B) →\rightarrow (t)
  • (C) →\rightarrow (r)
  • (D) →\rightarrow (r)