1) Cmr: $\forall \alpha$ ta có: $\sqrt{17}\leq \sqrt{cos^{2}\alpha -2cos\alpha +3}+\sqrt{cos^{2}\alpha +4cos\alpha +6}\leq \sqrt{2}+\sqrt{11}$
2) Cho $ a,b,c,d$ tm: $a^{2}+b^{2}=c^{2}+d^{2}=5.$. Cmr:
a) $\sqrt{5-a-2b}+\sqrt{5-c-2d}+\sqrt{5-ac-bd}\leq \frac{3\sqrt{30}}{2}$
b) $(5-a-2b)\left ( 5-c-2d \right )\left (5-ac-bd \right )\leq \left ( \frac{15}{2} \right )^{2}$
3) Cmr:
$\sqrt{2x^{2}-2x+1}+\sqrt{2x^{2}-(\sqrt{3}-1)x+1}+\sqrt{2x^{2}+(\sqrt{3}+1)x+1}\geq 3$