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

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

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

The type of surveying in which the curvature of the earth is taken into account is called