The greatest value of $c \in R$ for which the system of linear equations

$x – cy – cz = 0 \,\,;\,\, cx – y + cz = 0 \,\,;\,\, cx + cy – z = 0 $ has a non -trivial solution, is

$\left| {\,\begin{array}{*{20}{c}}{1 + i}&{1 – i}&i\\{1 – i}&i&{1 + i}\\i&{1 + i}&{1 – i}\end{array}\,} \right| = $

Let $\left| {\,\begin{array}{*{20}{c}}{6i}&{ – 3i}&1\\4&{3i}&{ – 1}\\{20}&3&i\end{array}\,} \right| = x + iy$, then

Let $a_1,a_2,a_3,….,a_{10}$ be in $G.P.$ with $a_i > 0$ for $i = 1, 2,….,10$ and $S$ be the set of pairs $(r,k), r, k \in N$ (the set of natural numbers) for which

$\left| {\begin{array}{*{20}{c}}
  {{{\log }_e}\,a_1^ra_2^k}&{{{\log }_e}\,a_2^ra_3^k}&{{{\log }_e}\,a_3^ra_4^k} \\
  {{{\log }_e}\,a_4^ra_5^k}&{{{\log }_e}\,a_5^ra_6^k}&{{{\log }_e}\,a_6^ra_7^k} \\ 
  {{{\log }_e}\,a_7^ra_8^k}&{{{\log }_e}\,a_8^ra_9^k}&{{{\log }_e}\,a_9^ra_{10}^k} 
\end{array}} \right| = 0$

Then the number of elements in $S$, is

$\left| {\,\begin{array}{*{20}{c}}1&5&\pi \\{{{\log }_e}e}&5&{\sqrt 5 }\\{{{\log }_{10}}10}&5&e\end{array}\,} \right| = $