The following system of equations $$$x+y+z=3,$$$ $$$x+2y+3z=4,$$$ $$$x+4y+kz=6$$$
will not have a unique solution for $$k$$ equal to

The eigenvalues of the matrix $$\left[ P \right] = \left[ {\matrix{ 4 & 5 \cr 2 & { - 5} \cr } } \right]$$ are

The product of matrices $${\left( {PQ} \right)^{ - 1}}P$$ is

A wastewater sample contains $$10^{-5.6}$$ mol/l of OH^- ions at $$25^o$$ C. The pH of this sample is