[tex]a) $ $ x^2+2x+2 = x^2+2x+1+1 = (x+1)^2+1 \ \textgreater \ 0, \forall x\in\mathbb_{R} $ \\ \\ b) $ $ x^4-4x^2+5 = x^2-4x^2+4+1 =(x^2-2)^2+1\ \textgreater \ 0, \forall x\in\mathbb_{R} $ \\ \\ c) $ x^2-x+1=x^2-2x+1+x = (x-1)^2 +x \ \textgreater \ 1, \forall x\in\mathbb_{R}[/tex]