a) cho a,b,c>0 thỏa a+b+c$\geq$$\frac{1}{a}+\frac{1}{b}+\frac{1}{c}$. chứng minh a+b+c$\geq \frac{3}{a+b+c}+\frac{2}{abc}$
b)cho a,b,c>0 và a+b+c=1. chứng minh $\sqrt{\frac{1}{a}-1}\sqrt{\frac{1}{b}-1}+\sqrt{\frac{1}{b}-1}\sqrt{\frac{1}{c}-1}+\sqrt{\frac{1}{c}-1}\sqrt{\frac{1}{a}-1}\geq 6$
c)cho a,b,c.0 thỏa abc=8. chứng minh $\frac{a^{2}}{\sqrt{(1+a^{3})(1+b^{3})}}+\frac{b^{2}}{\sqrt{(1+b^{3})(1+c^{3})}}+\frac{c^{2}}{\sqrt{(1+c^{3}(1+a^{3})}}\geq \frac{4}{3}$
d)cho x,y,z>0 thỏa x+y+z=3. chứng minh $\frac{x^{3}}{y^{3}+8}+\frac{y^{3}}{z^{3}+8}+\frac{z^3}{x^{3}+8}\geq \frac{1}{9}+\frac{2}{27}(xy+xz+yz)$
e)cho a,b,c>0.chứng minh $\frac{a}{a+\sqrt{(a+b)(a+c)}}+\frac{b}{b+\sqrt{(b+c)(b+a)}}+\frac{c}{c+\sqrt{(c+a)(c+b)}}\leq 1$
f)cho a,b,c,d thỏa $a^{2}+b^{2}+c^{2}+d^{2}=1$. chứng minh $(1-a)(1-b)(1-c)(1-d)\geq abcd$