Medium INTEGER +4 / -1 PYQ · JEE Mains 2021

Let $$S = \left\{ {n \in N\left| {{{\left( {\matrix{ 0 & i \cr 1 & 0 \cr } } \right)}^n}\left( {\matrix{ a & b \cr c & d \cr } } \right) = \left( {\matrix{ a & b \cr c & d \cr } } \right)\forall a,b,c,d \in R} \right.} \right\}$$, where i = $\sqrt { - 1}$. Then the number of 2-digit numbers in the set S is _____________.

Answer (integer) 11

Solution

Let $X = \left( {\matrix{ a &amp; b \cr c &amp; d \cr } } \right)$ &amp; $A = {\left( {\matrix{ 0 &amp; i \cr 1 &amp; 0 \cr } } \right)^n}$<br><br>$\Rightarrow$ AX = IX<br><br>$\Rightarrow$ A = I<br><br>$$ \Rightarrow {\left( {\matrix{ 0 &amp; i \cr 1 &amp; 0 \cr } } \right)^n} = I$$<br><br>$$ \Rightarrow {A^8} = \left[ {\matrix{ 1 &amp; 0 \cr 0 &amp; 1 \cr } } \right]$$<br><br>$\Rightarrow$ n is multiple of 8<br><br>So, number of 2 digit numbers in the set <br><br>S = 11 (16, 24, 32, .........., 96)

About this question

Subject: Mathematics · Chapter: Complex Numbers and Quadratic Equations · Topic: Complex Numbers and Argand Plane

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