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← JEE Main 2022 · 28 Jun · Shift 2
MathematicsInteger+4 / -1Question 27 of 90

JEE Main 2022 · 28 Jun · Shift 2, Mathematics question 27

Let A=(1+i1−i0)A=\begin{pmatrix}1+i & 1\\ -i & 0\end{pmatrix} where i=−1i=\sqrt{-1}. Then, the number of elements in the set {n∈{1,2,…,100}:An=A}\left\{n\in\{1,2,\dots,100\}:A^n=A\right\} is ______.

Answer and worked solution

Official answer: 25

Eigenvalues. Trace 1+i1+i, determinant ii: λ=1,i\lambda=1,i.

So An=AA^n=A exactly when in=ii^n=i, i.e. n≡1(mod4)n\equiv1\pmod4: 25 values in 1 to 100.

Answer: 25