Given A $A$ symmetric and semidefinite positive, for each x $x$ $$ x'Ax >= 1/||A|| ||Ax||^2\geq \frac{1}{\Vert A\Vert} \Vert Ax \Vert^2 $$
This inequality appears at page 24 of "Introduction to Optimization" from Boris T. Polyak. I haven't been able to prove it. Any idea?
Thanks, Giovanni

