For which value of kkk is the matrix A=(k11k)A = \begin{pmatrix} k & 1 \\ 1 & k \end{pmatrix}A=(k11k) not invertible?
k=0k = 0k=0
k=1k = 1k=1
k=1k = 1k=1 or k=−1k = -1k=−1
k=2k = 2k=2 or k=−2k = -2k=−2