$a)$ Cho $x^2+y^2+z^2=1. CMR: \frac{-1}{2}\leq xy+yz+xz\leq 1$
$b) a,b,c>0.CMR: a^2+b^2+c^2<2(ab+bc+ac)$
$c)a>0. CMR: \sqrt[3]{a}+\sqrt[3]{a^2}\leq 1+a$
$d)b,c>0. CMR: \frac{b+c}{bc}\geq \frac{4}{b+c}$
$e)a+b+c=1. a,b,c>0.CMR: b+c\geq 16abc$
$f)a+b=1. CMR: a^2+b^2\geq \frac{1}{2}$