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QJEEAdv24Ma4ab3: JEE Advanced 2024 Mathematics - Class 11 JEEAdv24Ma4ab3 Solution: Let , and . Then which of the following statements is (are) TRUE?

Let S={a+b2:a,b∈Z},T1={(−1+2)n:n∈N}S=\{a+b \sqrt{2}: a, b \in \mathbb{Z}\}, T_1=\left\{(-1+\sqrt{2})^n: n \in \mathbb{N}\right\} , and T2={(1+2)n:n∈N}T_2=\left\{(1+\sqrt{2})^n: n \in \mathbb{N}\right\} . Then which of the following statements is (are) TRUE?

Multiple Choice Options

  • A Z∪T1∪T2⊂S\mathbb{Z} \cup T_1 \cup T_2 \subset S
  • B T1∩(0,12024)=ϕT_1 \cap\left(0, \frac{1}{2024}\right)=\phi , where ϕ\phi denotes the empty set.
  • C T2∩(2024,∞)≠ϕT_2 \cap(2024, \infty) \neq \phi
  • D For any given a,b∈Z,cos⁡(π(a+b2))+isin⁡(π(a+b2))∈Za, b \in \mathbb{Z}, \cos (\pi(a+b \sqrt{2}))+i \sin (\pi(a+b \sqrt{2})) \in \mathbb{Z} if and only if b=0b=0 , where i=−1i=\sqrt{-1} .

Step-by-Step Verified Solution

(A)amp;(−1+2)n=m+2n,m,n∈Zamp;(1+2)n=m1+2n1, m1,n1∈Zamp;⇒Z∪T1∪T2⊆S\begin{aligned}\text{(A)} \quad & (-1+\sqrt{2})^{\mathrm{n}}=\mathrm{m}+\sqrt{2} \mathrm{n}, \mathrm{m}, \mathrm{n} \in \mathbb{Z} \\& (1+\sqrt{2})^{\mathrm{n}}=\mathrm{m}_1+\sqrt{2} \mathrm{n}_1, \mathrm{~m}_1, \mathrm{n}_1 \in \mathbb{Z} \\& \Rightarrow \mathbb{Z} \cup \mathrm{T}_1 \cup \mathrm{T}_2 \subseteq \mathrm{S}\end{aligned}

but b2∈Sb \sqrt{2} \in S for negative b∈Zb \in \mathbb{Z} .

So Z∪T1∪T2⊂S\quad \mathbb{Z} \cup T_1 \cup T_2 \subset \mathrm{S}

(B)amp;(2−1)n=1(2+1)nlt;12024amp;⇒2024lt;(2+1)n,∃n∈Namp;⇒T1∩(0,12024)≠ϕ\begin{aligned}\text{(B)}\quad& (\sqrt{2}-1)^{\mathrm{n}}=\frac{1}{(\sqrt{2}+1)^{\mathrm{n}}}<\frac{1}{2024} \\& \Rightarrow 2024<(\sqrt{2}+1)^{\mathrm{n}}, \exists \mathrm{n} \in \mathbb{N} \\& \Rightarrow \mathrm{T}_1 \cap\left(0, \frac{1}{2024}\right) \neq \phi\end{aligned}

(C)amp;(1+2)ngt;2024,∃n∈Namp;⇒T2∩(2024,∞)≠ϕ\begin{aligned}\text{(C)}\quad& (1+\sqrt{2})^{\mathrm{n}}>2024, \exists \mathrm{n} \in \mathbb{N} \\& \Rightarrow \mathrm{T}_2 \cap(2024, \infty) \neq \phi\end{aligned}

(D)\mathrm{(D)} sin⁡(π(a+b2)=0)⇒b=0,a∈Z\quad \sin (\pi(a+b \sqrt{2})=0) \Rightarrow b=0, a \in \mathbb{Z} .

⇒\Rightarrow Options (A), (C), (D) are Correct.

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