a)$\frac{2a^{2}+1}{\sqrt{4a^{2}+1}}\geq 1$ ; $a\geq 0$b)$a^{3}+b^{3}\geq ab(a+b)$ ; $a\geq 0 và b\geq 0$
c)$\frac{1}{a}+\frac{1}{b}+\frac{1}{c}\geq \frac{2}{a+b}+\frac{2}{b+c}+\frac{2}{a+c}$ ; $a\geq 0 ,b\geq 0$
d)$(1+a)(1+b)\geq (1+\sqrt{ab})^{2}$ ;$a\geq 0, b\geq 0$