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\(a_n=\sqrt{2+\frac{2}{n}+\frac{1}{n^2}}+\sqrt{2-\frac{2}{n}+\frac{1}{n^2}}\)
\(\Rightarrow\frac{1}{a_n}=\frac{1}{4}\left(\sqrt{\left(n+1\right)^2+n^2}-\sqrt{n^2+\left(n-1\right)^2}\right)\)
\(\Rightarrow S=\frac{1}{4}\left(\sqrt{2^2+1}-\sqrt{1^2+0}+\sqrt{3^2+2^2}-\sqrt{2^2+1}+...+\sqrt{21^2+20^2}-\sqrt{20^2+19^2}\right)\)
\(=\frac{1}{4}\left(\sqrt{21^2+20^2}-\sqrt{1}\right)=7\)
Mấy bài này đã có người làm rồi nhé bạn vào câu hỏi tương tự mà xem.
\(\sqrt{\left(1+\frac{1}{n}-\frac{1}{n+1}\right)^2-2\left(\frac{1}{n}-\frac{1}{n\left(n+1\right)}-\frac{1}{n+1}\right)}\)
=1+1/n-1/n+1
chúc bn hoc tốt
\(\sqrt{1+\frac{1}{n^2}+\frac{1}{\left(n+1\right)^2}}=1+\frac{1}{n}-\frac{1}{n+1}\)
\(\Rightarrow\frac{n^2\left(n+1\right)^2+\left(n+1\right)^2+n^2}{n^2\left(n+1\right)^2}=\frac{[\left(n+1\right)^2-n]^2}{n^2\left(n+1\right)^2}\)
\(\Rightarrow\left(n+1\right)^4+n^2=\left(n+1\right)^4-2\left(n+1\right)^2n+n^2\)
\(\Rightarrow0=-2\left(n+1\right)^2n\)
\(\Rightarrow\orbr{\begin{cases}\left(n+1\right)^2=0\\n=0\end{cases}}\Rightarrow\orbr{\begin{cases}n=-1\\n=0\end{cases}}\) mà \(n\inℕ^∗\)
=> n\(\in\varnothing\)
Xét dạng tổng quát có: \(\frac{1}{\sqrt{n+1}\left(n+1\right)+n\sqrt{n}}=\frac{1}{\left(\sqrt{n}+\sqrt{n+1}\right)\left[n-\sqrt{n\left(n+1\right)}+n+1\right]}\)
\(=\frac{\left(\sqrt{n+1}-\sqrt{n}\right)\left(\sqrt{n+1}+\sqrt{n}\right)}{\left(\sqrt{n}+\sqrt{n+1}\right)\left[n-\sqrt{n\left(n+1\right)}+n+1\right]}=\frac{\sqrt{n+1}-\sqrt{n}}{n+\left(n+1\right)-\sqrt{n\left(n+1\right)}}\)
\(< \frac{\sqrt{n+1}-\sqrt{n}}{2\sqrt{n\left(n+1\right)}-\sqrt{n\left(n+1\right)}}=\frac{\sqrt{n+1}-\sqrt{n}}{\sqrt{n\left(n+1\right)}}=\frac{1}{\sqrt{n}}-\frac{1}{\sqrt{n+1}}\)
Áp dụng vào bài toán ta có:
\(\frac{1}{2\sqrt{2}+1\sqrt{1}}< 1-\frac{1}{\sqrt{2}}\)
\(\frac{1}{3\sqrt{3}+2\sqrt{2}}< \frac{1}{\sqrt{2}}-\frac{1}{\sqrt{3}}\)
.....
\(\frac{1}{\left(n+1\right)\sqrt{n+1}+n\sqrt{n}}< \frac{1}{\sqrt{n}}-\frac{1}{\sqrt{n+1}}\)
Cộng vế theo vế =>\(VT< 1-\frac{1}{\sqrt{n+1}}\left(ĐPCM\right)\)
\(\sqrt{1+\frac{1}{n^2}+\frac{1}{\left(n+1\right)^2}}=\sqrt{\frac{n^2+\left(n+1\right)^2+\left(n^2+n\right)^2}{\left(n^2+n\right)^2}}\)
\(=\sqrt{\frac{2n^2+2n+1+\left(n^2+n\right)^2}{\left(n^2+n\right)^2}}=\sqrt{\frac{1+2\left(n^2+n\right)+\left(n^2+n\right)^2}{\left(n^2+n\right)^2}}\)
\(=\sqrt{\frac{\left(n^2+n+1\right)^2}{\left(n^2+n\right)^2}}=\frac{n^2+n+1}{n^2+n}=1+\frac{1}{n\left(n+1\right)}=1+\frac{1}{n}-\frac{1}{n+1}\)