\(\dfrac{\sqrt{3+2\sqrt{2}}+\sqrt{3-2\sqrt{2}}}{\sqrt{3+2\sqrt{2}}-\sqrt{3-2\sqrt{2}}}\)

\(=\dfrac{\sqrt{1+2\sqrt{2}+2}+\sqrt{2-2\sqrt{2}+1}}{\sqrt{1+2\sqrt{2}+2}-\sqrt{2-2\sqrt{2}+1}}\)

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

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

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

\(=\dfrac{2\sqrt{2}}{1+\sqrt{2}-\sqrt{2}+1}\)

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

Kết luận: ...

a. \(\left(\sqrt{28}-2\sqrt{14}+\sqrt{7}\right)\sqrt{7}+7\sqrt{8}=\left(2\sqrt{7}-2\sqrt{14}+\sqrt{7}\right)\sqrt{7}+14\sqrt{2}=14-14\sqrt{2}+7+14\sqrt{2}=21\)

b. \(\dfrac{\sqrt{15}-\sqrt{5}}{\sqrt{3}-1}-\dfrac{5-2\sqrt{5}}{2\sqrt{5}-4}=\dfrac{\sqrt{5}\left(\sqrt{3}-1\right)}{\sqrt{3}-1}-\dfrac{\sqrt{5}\left(\sqrt{5}-2\right)}{2\left(\sqrt{5}-2\right)}=\sqrt{5}-\dfrac{\sqrt{5}}{2}=\dfrac{2\sqrt{5}-\sqrt{5}}{2}=\dfrac{\sqrt{5}}{2}\)

c. \(\dfrac{\sqrt{6}+\sqrt{14}}{2\sqrt{3}+\sqrt{28}}=\dfrac{\sqrt{6}+\sqrt{14}}{2\sqrt{3}+2\sqrt{7}}=\dfrac{\sqrt{2}\left(\sqrt{3}+\sqrt{7}\right)}{2\left(\sqrt{3}+\sqrt{7}\right)}=\dfrac{\sqrt{2}}{2}\)

\(\left(\dfrac{\sqrt{x}}{\sqrt{x}+2}-\dfrac{3}{2-\sqrt{x}}+\dfrac{3\sqrt{x}-2}{x-2}\right):\left(\dfrac{\sqrt{x}+3}{\sqrt{x}-2}+\dfrac{2\sqrt{x}}{2\sqrt{x}-x}\right)=\dfrac{x-2\sqrt{x}+3\sqrt{x}+6+3\sqrt{x}-2}{\left(\sqrt{x}+2\right)\left(\sqrt{x}-2\right)}:\dfrac{\sqrt{x}+1}{\sqrt{x}-2}=\dfrac{\left(\sqrt{x}+2\right)^2}{\left(\sqrt{x}+2\right)\left(\sqrt{x}-2\right)}.\dfrac{\sqrt{x}-2}{\sqrt{x}+1}=\dfrac{\sqrt{x}+2}{\sqrt{x}+1}\)

Mấy bài này rất dài , đăng từ từ thôi nhé bạn .

\(1.\dfrac{\sqrt{30}-\sqrt{2}}{\sqrt{8}-\sqrt{15}}-\sqrt{8-\sqrt{49+8\sqrt{3}}}=\dfrac{\sqrt{60}-\sqrt{4}}{\sqrt{16-2\sqrt{15}}}-\sqrt{8-\sqrt{48+2.4\sqrt{3}+1}}=\dfrac{2\left(\sqrt{15}-1\right)}{\sqrt{\left(\sqrt{15}-1\right)^2}}-\sqrt{8-|4\sqrt{3}+1|}=2-\sqrt{4-2.2\sqrt{3}+3}=2-|2-\sqrt{3}|=\sqrt{3}\)

\(2.\dfrac{2+\sqrt{3}}{\sqrt{2}+\sqrt{2+\sqrt{3}}}+\dfrac{2-\sqrt{3}}{\sqrt{2}-\sqrt{2-\sqrt{3}}}=\dfrac{2\sqrt{2}+\sqrt{6}}{\sqrt{4}+\sqrt{4+2\sqrt{3}}}+\dfrac{2\sqrt{2}-\sqrt{6}}{\sqrt{4}-\sqrt{4-2\sqrt{3}}}=\dfrac{2\sqrt{2}+\sqrt{6}}{2+\sqrt{\left(\sqrt{3}+1\right)^2}}+\dfrac{2\sqrt{2}-\sqrt{6}}{2-\sqrt{\left(\sqrt{3}-1\right)^2}}=\dfrac{2\sqrt{2}+\sqrt{6}}{2+|\sqrt{3}+1|}+\dfrac{2\sqrt{2}-\sqrt{6}}{2-|\sqrt{3}-1|}=\dfrac{2\sqrt{2}-\sqrt{6}}{3-\sqrt{3}}+\dfrac{2\sqrt{2}+\sqrt{6}}{3+\sqrt{3}}=\dfrac{12\sqrt{2}-2\sqrt{18}}{9-3}=\dfrac{12\sqrt{2}-6\sqrt{2}}{6}=\dfrac{6\sqrt{2}}{6}=\sqrt{2}\)

\(3.\dfrac{\sqrt{2}}{2\sqrt{2}+\sqrt{3+\sqrt{5}}}+\dfrac{\sqrt{2}}{2\sqrt{2}-\sqrt{3-\sqrt{5}}}=\dfrac{2}{4+\sqrt{5+2\sqrt{5}+1}}+\dfrac{2}{4-\sqrt{5-2\sqrt{5}+1}}=\dfrac{2}{4+|\sqrt{5}+1|}+\dfrac{2}{4-|\sqrt{5}-1|}=\dfrac{2}{\sqrt{5}+5}+\dfrac{2}{5-\sqrt{5}}=\dfrac{10-2\sqrt{5}+10+2\sqrt{5}}{20}=\dfrac{20}{20}=1\)

1)Để căn có nghĩa \(\Leftrightarrow\dfrac{-a}{3}\ge0\Leftrightarrow a\le0\)

Vậy...

2)Để căn có nghĩa \(\Leftrightarrow\left\{{}\begin{matrix}\dfrac{a^2+1}{1-3a}\ge0\\1-3a\ne0\end{matrix}\right.\)\(\Leftrightarrow\left\{{}\begin{matrix}1-3a>0\left(vìa^2+1>0\right)\\1-3a\ne0\end{matrix}\right.\)

\(\Leftrightarrow1-3a>0\Leftrightarrow3a< 1\Leftrightarrow a< \dfrac{1}{3}\)

Vậy...

3)Để căn có nghĩa 

\(\Leftrightarrow a^2-6a+10\ge0\Leftrightarrow\left(a^2-6a+9\right)+1\ge0\Leftrightarrow\left(a-3\right)^2+1\ge0\left(lđ;\forall a\right)\)

Vậy căn luôn có nghĩa với mọi a

4)Để căn có nghĩa \(\Leftrightarrow\left\{{}\begin{matrix}\dfrac{a-1}{a+2}\ge0\\a+2\ne0\end{matrix}\right.\) \(\Leftrightarrow\left\{{}\begin{matrix}\left[{}\begin{matrix}\left\{{}\begin{matrix}a-1\ge0\\a+2>0\end{matrix}\right.\\\left\{{}\begin{matrix}a-1\le0\\a+2< 0\end{matrix}\right.\end{matrix}\right.\\a+2\ne0\end{matrix}\right.\)\(\Leftrightarrow\left[{}\begin{matrix}\left\{{}\begin{matrix}a\ge1\\a>-2\end{matrix}\right.\\\left\{{}\begin{matrix}a\le1\\a< -2\end{matrix}\right.\end{matrix}\right.\)\(\Leftrightarrow\left[{}\begin{matrix}a\ge1\\a< -2\end{matrix}\right.\)

Vậy...

a: Ta có: \(\dfrac{2}{\sqrt{3}+1}+\dfrac{2}{2-\sqrt{3}}\)

\(=\sqrt{3}-1+2+\sqrt{3}\)

\(=2\sqrt{3}+1\)

b: Ta có: \(\dfrac{4}{\sqrt{5}+2}+\dfrac{2}{3+\sqrt{5}}\)

\(=4\sqrt{5}-8+\dfrac{3}{2}-\dfrac{\sqrt{5}}{2}\)

\(=-\dfrac{13}{2}+\dfrac{7}{2}\sqrt{5}\)

Bạn cần giúp nhanh nhưng lại không ghi đầy đủ đề bài?

\(A=\dfrac{\sqrt{a+1}}{\sqrt{a+1}.\sqrt{a-1}-\sqrt{a}.\sqrt{a+1}}+\dfrac{1}{\sqrt{a-1}+\sqrt{a}}+\dfrac{a\left(\sqrt{a}-1\right)}{\sqrt{a}-1}\\ =\dfrac{\sqrt{a+1}}{\sqrt{a+1}\left(\sqrt{a-1}-\sqrt{a}\right)}+\dfrac{1}{\sqrt{a-1}+\sqrt{a}}+a\\ =\dfrac{1}{\sqrt{a-1}-\sqrt{a}}+\dfrac{1}{\sqrt{a-1}+\sqrt{a}}+a\\ =\dfrac{\sqrt{a-1}+\sqrt{a}}{a-1-a}+\dfrac{\sqrt{a-1}-\sqrt{a}}{a-1-a}+a\\ =\dfrac{2\sqrt{a-1}}{-1}+a\\ =-2\sqrt{a-1}+a.\)

Bạn ghi thiếu đề hoặc đề sai không vậy??

Biểu thức không bằng một giá trị nào đó thì sao tìm x được :>

a) Để \(\dfrac{\sqrt{x}}{\sqrt{x}-3}\) có nghĩa khì \(x\ge0;x\ne9\)

b) Để \(\dfrac{\sqrt{x}}{\sqrt{x}+6}\) có nghĩa khi \(x\ge0\)