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JEE Advanced 2026Paper 2
Exam
JEE Advanced
Year
2026
Paper
Paper 2
Subject
Mathematics
Chapter
Matrices

JEE Advanced 2026 · Paper 2 · Mathematics · Matrices

Let ℝ denote the set of all real numbers and let i = -1 . Consider...

Let R\mathbb{R} denote the set of all real numbers and let i=−1i = \sqrt{-1}. Consider the matrices
S=[0−110]S = \begin{bmatrix} 0 & -1 \\ 1 & 0 \end{bmatrix} and T=[1101]T = \begin{bmatrix} 1 & 1 \\ 0 & 1 \end{bmatrix}.
Let a,b,c,da, b, c, d be real numbers such that
ST=[abcd]ST = \begin{bmatrix} a & b \\ c & d \end{bmatrix}.
Let
H={x+iy:x,y∈R and y>0}H = \{x + iy : x, y \in \mathbb{R} \text{ and } y > 0\}.
Then which of the following statements is (are) TRUE ?

Options

  1. Ab+iad+ic=i\dfrac{b + ia}{d + ic} = i
  2. BIf ω=−1+i32\omega = \dfrac{-1 + i\sqrt{3}}{2}, then aω+bcω+d=ω\dfrac{a\omega + b}{c\omega + d} = \omega
  3. CIf mm is an integer greater than 22 such that (ST)2=(ST)m(ST)^2 = (ST)^m, then mm is an integer multiple of 88
  4. DIf z∈Hz \in H, then az+bcz+d∈H\dfrac{az + b}{cz + d} \in H

Correct answer

B. If ω=−1+i32\omega = \dfrac{-1 + i\sqrt{3}}{2}, then aω+bcω+d=ω\dfrac{a\omega + b}{c\omega + d} = \omega

Step-by-step solution

We are given the matrices S=[0−110]S = \begin{bmatrix} 0 & -1 \\ 1 & 0 \end{bmatrix} and T=[1101]T = \begin{bmatrix} 1 & 1 \\ 0 & 1 \end{bmatrix}.

First, we compute the product STST:
ST=[0−110][1101]=[0−111]ST = \begin{bmatrix} 0 & -1 \\ 1 & 0 \end{bmatrix} \begin{bmatrix} 1 & 1 \\ 0 & 1 \end{bmatrix} = \begin{bmatrix} 0 & -1 \\ 1 & 1 \end{bmatrix}

Comparing this with ST=[abcd]ST = \begin{bmatrix} a & b \\ c & d \end{bmatrix}, we get a=0a = 0, b=−1b = -1, c=1c = 1, and d=1d = 1.

Evaluating Option (A):
b+iad+ic=−1+i(0)1+i(1)=−11+i=−1(1−i)(1+i)(1−i)=−1+i2\dfrac{b + ia}{d + ic} = \dfrac{-1 + i(0)}{1 + i(1)} = \dfrac{-1}{1 + i} = \dfrac{-1(1 - i)}{(1 + i)(1 - i)} = \dfrac{-1 + i}{2}
This is not equal to ii, so statement (A) is FALSE.

Evaluating Option (B):
Given ω=−1+i32\omega = \dfrac{-1 + i\sqrt{3}}{2}, which is a complex cube root of unity, satisfying ω2+ω+1=0\omega^2 + \omega + 1 = 0 and ω3=1\omega^3 = 1.
aω+bcω+d=0⋅ω−11⋅ω+1=−1ω+1\dfrac{a\omega + b}{c\omega + d} = \dfrac{0 \cdot \omega - 1}{1 \cdot \omega + 1} = \dfrac{-1}{\omega + 1}
Since ω+1=−ω2\omega + 1 = -\omega^2, we have:
−1−ω2=1ω2=ω3ω2=ω\dfrac{-1}{-\omega^2} = \dfrac{1}{\omega^2} = \dfrac{\omega^3}{\omega^2} = \omega
Thus, statement (B) is TRUE.

Evaluating Option (C):
The characteristic equation of STST is λ2−Tr(ST)λ+det⁡(ST)=0\lambda^2 - \text{Tr}(ST)\lambda + \det(ST) = 0, which gives λ2−λ+1=0\lambda^2 - \lambda + 1 = 0.
By the Cayley-Hamilton theorem, (ST)2−ST+I=0(ST)^2 - ST + I = 0.
Multiplying by (ST+I)(ST + I), we get (ST)3+I=0⇒(ST)3=−I(ST)^3 + I = 0 \Rightarrow (ST)^3 = -I.
Squaring both sides gives (ST)6=I(ST)^6 = I.
We are given (ST)2=(ST)m(ST)^2 = (ST)^m, which implies (ST)m−2=I(ST)^{m-2} = I.
This means m−2m - 2 must be a multiple of 66, so m=6k+2m = 6k + 2 for some integer kk.
For k=1k = 1, m=8m = 8 (which is a multiple of 88).
For k=2k = 2, m=14m = 14 (which is NOT a multiple of 88).
Thus, statement (C) is FALSE.

Evaluating Option (D):
For z∈Hz \in H, let z=x+iyz = x + iy where y>0y > 0.
az+bcz+d=−1z+1=−1(x+1)+iy\dfrac{az + b}{cz + d} = \dfrac{-1}{z + 1} = \dfrac{-1}{(x + 1) + iy}
Multiplying the numerator and denominator by the conjugate (x+1)−iy(x + 1) - iy:
−1(x+1)+iy×(x+1)−iy(x+1)−iy=−(x+1)+iy(x+1)2+y2\dfrac{-1}{(x + 1) + iy} \times \dfrac{(x + 1) - iy}{(x + 1) - iy} = \dfrac{-(x + 1) + iy}{(x + 1)^2 + y^2}
The imaginary part of this new complex number is y(x+1)2+y2\dfrac{y}{(x + 1)^2 + y^2}.
Since y>0y > 0 and (x+1)2+y2>0(x + 1)^2 + y^2 > 0, the imaginary part is strictly positive.
Therefore, az+bcz+d∈H\dfrac{az + b}{cz + d} \in H, making statement (D) TRUE.

Answer: If ω=−1+i32\omega = \dfrac{-1 + i\sqrt{3}}{2}, then aω+bcω+d=ω\dfrac{a\omega + b}{c\omega + d} = \omega; If z∈Hz \in H, then az+bcz+d∈H\dfrac{az + b}{cz + d} \in H
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