Đặt \(S=\frac{1}{\sqrt{n^2+1}}+\frac{1}{\sqrt{n^2+2}}+...+\frac{1}{\sqrt{n^2+n}}\)
\(n^2+1< n^2+2< ...< n^2+n\)
\(\Rightarrow\frac{1}{\sqrt{n^2+1}}>\frac{1}{\sqrt{n^2+2}}>...>\frac{1}{\sqrt{n^2+n}}\)
\(\Rightarrow\frac{n}{\sqrt{n^2+1}}>\frac{1}{\sqrt{n^2+1}}+\frac{1}{\sqrt{n^2+2}}+...+\sqrt{n^2+n}\)
\(\frac{n}{\sqrt{n^2+n}}< \frac{1}{\sqrt{n^2+1}}+\frac{1}{\sqrt{n^2+n}}+...+\frac{1}{\sqrt{n^2+n}}\)
\(\Rightarrow\frac{n}{\sqrt{n^2+n}}< S< \frac{n}{\sqrt{n^2+1}}\)
Mà \(lim\frac{n}{\sqrt{n^2+1}}=lim\frac{n}{\sqrt{n^2+n}}=1\)
\(\Rightarrow lim\left(S\right)=1\) theo nguyên lý kẹp
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