Chứng minh bất đẳng thức:
a) $\frac{1}{\sqrt{1} }+\frac{1}{\sqrt{2}}+...+\frac{1}{\sqrt{n}} \geq 2( \sqrt{n+1}-1) b) \frac{1}{1^{3}}+\frac{1}{2^{3}}+...+\frac{1}{n^{3} } \leq 2 $
c) $2<\frac{1}{ \sqrt{1}+ \sqrt{2}}+ \frac{1}{ \sqrt{2}+ \sqrt{3}}+...+\frac{1}{ \sqrt{11}+ \sqrt{12}}<3$
|