Bạn xem lại đề nhé. Theo mình nghĩ thì không có căn 4 ở sau dấu.... Đây là vô hạn mà.

cảm ơn phương pháp của bạn nhiều nhé, nhờ bạn mà mình làm đc rồi ^^

Bài này may mình có thi qua rùi.

Đặt

\(A=\sqrt{4+\sqrt{4+\sqrt{4+...+\sqrt{4}}}}>0\)

=> \(A^2=4+\sqrt{4+\sqrt{4+...+\sqrt{4}}}\)

=> A² - A = 4           

=> A² - A - 4 = 0

Giải phương trình được 2 nghiệm:

\(A_1=\frac{1+\sqrt{17}}{2}\)

\(A_2=\frac{1-\sqrt{17}}{2}< 0\)( loại vì A>0)

Vậy \(A=\frac{1+\sqrt{17}}{2}< \frac{1+\sqrt{25}}{2}=\frac{1+5}{2}=3\)

Kết luận: \(\sqrt{4+\sqrt{4+\sqrt{4+...+\sqrt{4}}}}< 3\)

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Chắc bạn ko hiểu chỗ A² - A = 4 nhỉ?

SO SÁNH√4+√4+√4+...+√4

 VỚI 3

SO SÁNH√4+√4+√4+...+√4

 VỚI 3

\(\left(\sqrt{4+\sqrt{5+\sqrt{6}}}\right)^2=4+\sqrt{5+\sqrt{6}};3^2=9=4+5\left(1\right)\\ \left(\sqrt{5+\sqrt{6}}\right)^2=5+\sqrt{6};5^2=25=5+20\left(2\right)\\ \left(\sqrt{6}\right)^2=6;20^2=400\\ \Leftrightarrow\sqrt{6}< 20\)

Thay vào \(\left(2\right)\Leftrightarrow\sqrt{5+\sqrt{6}}< 5\)

Thay vào \(\left(1\right)\Leftrightarrow\sqrt{4+\sqrt{5+\sqrt{6}}}< 3\)

Ta có: 

\(R=\)\(\dfrac{3+\sqrt{5}}{2\sqrt{2}+\sqrt{3+\sqrt{5}}}+\dfrac{3-\sqrt{5}}{2\sqrt{2}-\sqrt{3-\sqrt{5}}}\)

\(=\)\(\dfrac{\sqrt{10}+3\sqrt{2}}{5+\sqrt{5}}+\dfrac{\sqrt{10}-3\sqrt{2}}{5-\sqrt{5}}\)

\(=\dfrac{4\sqrt{2}}{\sqrt{5}\left(\sqrt{5}+1\right)\left(\sqrt{5}-1\right)}\)

\(=\dfrac{4\sqrt{2}}{4\sqrt{5}}=\sqrt{\dfrac{2}{5}}\)

Làm câu S tương tự như này rồi đối chiếu kết quả nha

Ta có:

\(\frac{1-\sqrt{n}+\sqrt{n+1}}{1+\sqrt{n}+\sqrt{n+1}}=\frac{\left(1-\sqrt{n}+\sqrt{n+1}\right)^2}{\left(1+\sqrt{n}+\sqrt{n+1}\right)\left(1-\sqrt{n}+\sqrt{n+1}\right)}=\frac{2n+2-2\sqrt{n}+2\sqrt{n+1}-2\sqrt{n\left(n+1\right)}}{2\left(1+\sqrt{n+1}\right)}\)

\(=\frac{\left[2n+2-2\sqrt{n}+2\sqrt{n+1}-2\sqrt{n\left(n+1\right)}\right]\left(1-\sqrt{n+1}\right)}{2\left(1+\sqrt{n+1}\right)\left(1-\sqrt{n+1}\right)}=\frac{-2n\sqrt{n+1}+2n\sqrt{n}}{-2n}=\sqrt{n+1}-\sqrt{n}\)

Suy ra:

\(Q=\sqrt{3}-\sqrt{2}+\sqrt{4}-\sqrt{3}+...+\sqrt{2017}-\sqrt{2016}=\sqrt{2017}-\sqrt{2}< \sqrt{2017}-1=R\)

Vậy Q < R.

Sửa đề: \(S=\frac{4+\sqrt7}{3\sqrt2+\sqrt{4+\sqrt7}}+\frac{4-\sqrt7}{3\sqrt2-\sqrt{4-\sqrt7}}\)

\(=\frac{\sqrt2\left(4+\sqrt7\right)}{6+\sqrt{8+2\sqrt7}}+\frac{\sqrt2\left(4-\sqrt7\right)}{6-\sqrt{8-2\sqrt7}}\)

\(=\frac{\sqrt2\left(4+\sqrt7\right)}{6+\sqrt{\left(\sqrt7+1\right)^2}}+\frac{\sqrt2\left(4-\sqrt7\right)}{6-\sqrt{\left(\sqrt7-1\right)^2}}\)

\(=\frac{\sqrt2\left(4+\sqrt7\right)}{6+\sqrt7+1}+\frac{\sqrt2\left(4-\sqrt7\right)}{6-\left(\sqrt7-1\right)}=\frac{\sqrt2\left(4+\sqrt7\right)}{7+\sqrt7}+\frac{\sqrt2\left(4-\sqrt7\right)}{7-\sqrt7}\)

\(=\frac{1}{\sqrt2}\cdot\left\lbrack\frac{2\left(4+\sqrt7\right)}{\sqrt7\left(\sqrt7+1\right)}+\frac{2\left(4-\sqrt7\right)}{\sqrt7\left(\sqrt7-1\right)}\right\rbrack\)

\(=\frac{1}{\sqrt2}\cdot\left\lbrack\frac{8+2\sqrt7}{\sqrt7\left(\sqrt7+1\right)}+\frac{8-2\sqrt7}{\sqrt7\left(\sqrt7-1\right)}\right\rbrack\)

\(=\frac{1}{\sqrt2}\cdot\left\lbrack\frac{\left(\sqrt7+1\right)^2}{\sqrt7\left(\sqrt7+1\right)}+\frac{\left(\sqrt7-1\right)^2}{\sqrt7\left(\sqrt7-1\right)}\right\rbrack=\frac{1}{\sqrt2}\cdot\frac{\sqrt7+1+\sqrt7-1}{\sqrt7}=\frac{2\sqrt7}{\sqrt2\cdot\sqrt7}=\sqrt2\)

Ta có: \(R=\frac{3+\sqrt5}{2\sqrt2+\sqrt{3+\sqrt5}}+\frac{3-\sqrt5}{2\sqrt2-\sqrt{3-\sqrt5}}\)

\(=\frac{\sqrt2\left(3+\sqrt5\right)}{4+\sqrt{6+2\sqrt5}}+\frac{\sqrt2\left(3-\sqrt5\right)}{4-\sqrt{6-2\sqrt5}}\)

\(=\frac{\sqrt2\left(3+\sqrt5\right)}{4+\sqrt5+1}+\frac{\sqrt2\left(3-\sqrt5\right)}{4-\sqrt5+1}\)

\(=\frac{\sqrt2\left(3+\sqrt5\right)}{5+\sqrt5}+\frac{\sqrt2\left(3-\sqrt5\right)}{5-\sqrt5}=\sqrt2\cdot\left\lbrack\frac{3+\sqrt5}{\sqrt5\left(\sqrt5+1\right)}+\frac{3-\sqrt5}{\sqrt5\left(\sqrt5-1\right)}\right\rbrack\)

\(=\frac{\sqrt2}{2}\cdot\left\lbrack\frac{6+2\sqrt5}{\sqrt5\left(\sqrt5+1\right)}+\frac{6-2\sqrt5}{\sqrt5\left(\sqrt5-1\right)}\right\rbrack\)

\(=\frac{\sqrt2}{2}\cdot\left\lbrack\frac{\left(\sqrt5+1\right)^2}{\sqrt5\left(\sqrt5+1\right)}+\frac{\left(\sqrt5-1\right)^2}{\sqrt5\left(\sqrt5-1\right)}\right\rbrack=\frac{\sqrt2}{2}\cdot\frac{\sqrt5+1+\sqrt5-1}{\sqrt5}=\frac{\sqrt2}{2}\cdot2=\sqrt2\)

Do đó: R=S

100 dấu căn nha

\(\sqrt{4+\sqrt{4+\sqrt{4+\sqrt{4+...}}}}< \sqrt{6+\sqrt{6+\sqrt{6+...\sqrt{6+\sqrt{9}}}}}\)(100 dấu căn)

=> \(VT< \sqrt{6+\sqrt{6+\sqrt{6+...\sqrt{6+3}}}}=\sqrt{6+\sqrt{6+\sqrt{6+..\sqrt{6+\sqrt{9}}}}}\)(99 dấu căn)

=> \(VT< \sqrt{6+3}=3\)