đặt \(ax^3=by^3=cz^3=k^3\) thì \(a=\frac{k^3}{x^3};b=\frac{k^3}{y^3};c=\frac{k^3}{z^3}\)

\(\sqrt[3]{a}+\sqrt[3]{b}+\sqrt[3]{c}=\frac{k}{x}+\frac{k}{y}+\frac{k}{z}=k\left(\frac{1}{x}+\frac{1}{y}+\frac{1}{z}\right)=k\)

Mặt khác : \(ax^2+by^2+cz^2=\frac{ax^3}{x}+\frac{by^3}{y}+\frac{cz^3}{z}=\frac{k^3}{x}+\frac{k^3}{y}+\frac{k^3}{z}=k^3\)

\(\Rightarrow\sqrt[3]{ax^2+by^2+cz^2}=k\)

Do đó , ta có đpcm

\(\sin90\)

\(\frac{1}{\sqrt{13-\sqrt{48}}}=\frac{1}{\sqrt{12+1+2\cdot2\sqrt{3}}}=\frac{1}{2\sqrt{3}+1}=\frac{-1+2\sqrt{3}}{11}\)\

Câu b nè: 

\(B=\frac{2}{\left(\sqrt[3]{2}\right)^2+\sqrt[3]{2}+\left(\sqrt[3]{2}\right)^3}\)

Đặt: \(\sqrt[3]{2}=a\)

=> \(B=\frac{a^3}{a^3+a^2+a}=\frac{a^2}{a^2+a+1}=\frac{a^2\left(a-1\right)}{\left(a^2+a+1\right)\left(a-1\right)}=\frac{a^3-a^2}{a^3-1}=\frac{2-\sqrt[3]{4}}{2-1}=2-\sqrt[3]{4}\)

Vậy \(B=2-\sqrt[3]{4}\)

Tham khảo tại đây : Câu hỏi của Huỳnh Kim Bích Ngọc - Toán lớp 8 - Học toán với OnlineMath

\(A=\left(\frac{\sqrt{a}}{\sqrt{a}-1}-\frac{1}{a-\sqrt{a}}\right):\left(\frac{1}{\sqrt{a}+1}+\frac{2}{a-1}\right)\left(a>0;a\ne1\right)\)

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

\(A=\frac{a-1}{\sqrt{a}\left(\sqrt{a}-1\right)}:\frac{\sqrt{a}+1}{a-1}\)

\(A=\frac{\sqrt{a}+1}{\sqrt{a}}:\frac{1}{\sqrt{a}-1}\)

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

Vậy..............
\(B=\left(\frac{\sqrt{a}}{\sqrt{a}+1}-\frac{\sqrt{a}}{\sqrt{a}-1}+\frac{1}{a-1}\right):\frac{a}{2+2\sqrt{a}}\)( điều kiện như trên )

\(B=\frac{\sqrt{a}\left(\sqrt{a}-1\right)-\sqrt{a}\left(\sqrt{a}+1\right)+1}{a-1}:\frac{a}{2\left(1+\sqrt{a}\right)}\)

\(B=\frac{a-\sqrt{a}-a-\sqrt{a}+1}{a-1}:\frac{a}{\left(\sqrt{a}+1\right).2}\)

\(B=\frac{1-2\sqrt{a}}{\left(\sqrt{a}-1\right)\left(\sqrt{a}+1\right)}.\frac{\left(\sqrt{a}+1\right).2}{a}\)

\(B=\frac{2\left(1-2\sqrt{a}\right)}{a\left(\sqrt{a}-1\right)}\)

Vậy.........

_Minh ngụy_

\(P=\left(1+\frac{\sqrt{a}-1}{a-\sqrt{a}}\right):\left(\frac{a+\sqrt{a}}{a-1}+\frac{\sqrt{a}}{a-\sqrt{a}}\right)\left(a>0;a\ne1\right)\)

\(P=\frac{a-\sqrt{a}+\sqrt{a}-1}{\sqrt{a}\left(\sqrt{a}-1\right)}:\left[\frac{\sqrt{a}\left(\sqrt{a}+1\right)}{\left(\sqrt{a}-1\right)\left(\sqrt{a}+1\right)}+\frac{\sqrt{a}}{\sqrt{a}\left(\sqrt{a}-1\right)}\right]\)

\(P=\frac{a-1}{\sqrt{a}\left(\sqrt{a}-1\right)}:\left(\frac{\sqrt{a}}{\sqrt{a}-1}+\frac{1}{\sqrt{a}-1}\right)\)

\(P=\frac{\left(\sqrt{a}-1\right)\left(\sqrt{a}+1\right)}{\sqrt{a}\left(\sqrt{a}-1\right)}:\left(\frac{\sqrt{a}+1}{\sqrt{a}-1}\right)\)

\(P=\frac{\sqrt{a}+1}{\sqrt{a}}:\frac{\sqrt{a}+1}{\sqrt{a}-1}\)

\(P=\frac{\sqrt{a}+1}{\sqrt{a}}.\frac{\sqrt{a}-1}{\sqrt{a}+1}=\frac{\sqrt{a}-1}{\sqrt{a}}\)

Vậy .............

_Minh ngụy_

\(P=\left(1+\frac{\sqrt{a}-1}{a-\sqrt{a}}\right):\left(\frac{a+\sqrt{a}}{a-1}+\frac{\sqrt{a}}{a-\sqrt{a}}\right)\text{ (ĐKXĐ: }x\ne1;x\ne0\text{ )}\)

\(P=\left(\frac{a-\sqrt{a}}{a-\sqrt{a}}+\frac{\sqrt{a}-1}{a-\sqrt{a}}\right):\left(\frac{\left(a+\sqrt{a}\right)\left(a-\sqrt{a}\right)}{\left(a-1\right)\left(a-\sqrt{a}\right)}+\frac{\sqrt{a}\left(a-1\right)}{\left(a-\sqrt{a}\right)\left(a-1\right)}\right)\)

\(P=\frac{a-\sqrt{a}+\sqrt{a}-1}{a-\sqrt{a}}:\left(\frac{a^2-a}{\left(a-1\right)\left(a-\sqrt{a}\right)}+\frac{\sqrt{a}\left(a-1\right)}{\left(a-\sqrt{a}\right)\left(a-1\right)}\right)\)

\(P=\frac{a-1}{a-\sqrt{a}}:\frac{a\left(a-1\right)+\sqrt{a}\left(a-1\right)}{\left(a-1\right)\left(a-\sqrt{a}\right)}\)

\(P=\frac{a-1}{a-\sqrt{a}}:\frac{\left(a-1\right)\left(a+\sqrt{a}\right)}{\left(a-1\right)\left(a-\sqrt{a}\right)}\)

\(P=\frac{a-1}{a-\sqrt{a}}:\frac{a+\sqrt{a}}{a-\sqrt{a}}\)

\(P=\frac{a-1}{a-\sqrt{a}}\times\frac{a-\sqrt{a}}{a+\sqrt{a}}\)

\(P=\frac{a-1}{a+\sqrt{a}}\)

\(\frac{1}{y\left(x-y\right)}-\frac{1}{x\left(x-y\right)}=\frac{x-y}{y\left(x-2\right)x}=\frac{1}{yx}\)