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6 tháng 8 2020

A=\(\frac{u-v}{\sqrt{u}+\sqrt{v}}-\frac{\sqrt{u^3}+\sqrt{v^3}}{u-v}=\frac{\left(\sqrt{u}-\sqrt{v}\right)\left(\sqrt{u}+\sqrt{v}\right)}{\sqrt{u}+\sqrt{v}}-\frac{\left(\sqrt{u}+\sqrt{v}\right)\left(u-\sqrt{u}\sqrt{v}+v\right)}{\left(\sqrt{u}+\sqrt{v}\right)\left(\sqrt{u}-\sqrt{v}\right)}\)

\(=\sqrt{u}-\sqrt{v}-\frac{u-\sqrt{uv}+v}{\sqrt{u}-\sqrt{v}}=\frac{u-2\sqrt{uv}+v-u+\sqrt{uv}-v}{\sqrt{u}-\sqrt{v}}=\frac{-\sqrt{uv}}{\sqrt{u}-\sqrt{v}}\)

6 tháng 8 2020

bạn vào thống kê của mình có link tham khảo 

Câu hỏi của Duy Saker Hy - Toán lớp 9 - Học toán với OnlineMath

16 tháng 8 2018

\(A=\dfrac{u-v}{\sqrt{u}+\sqrt{v}}-\dfrac{\sqrt{u^3}+\sqrt{v^3}}{u-v}\)

\(=\sqrt{u}-\sqrt{v}-\dfrac{u\sqrt{u}+v\sqrt{v}}{\left(\sqrt{u}-\sqrt{v}\right)\left(\sqrt{u}+\sqrt{v}\right)}\)

\(=\sqrt{u}-\sqrt{v}-\dfrac{u-\sqrt{uv}+v}{\left(\sqrt{u}-\sqrt{v}\right)\left(\sqrt{u}+\sqrt{v}\right)}\)

\(=\sqrt{u}-\sqrt{v}-\dfrac{u-\sqrt{uv}+v}{\sqrt{u}-\sqrt{v}}\)

\(=\dfrac{\left(\sqrt{u}-\sqrt{v}\right)\sqrt{u}-\left(\sqrt{u}-\sqrt[]{v}\right)\sqrt{v}-\left(u-\sqrt{uv}+v\right)}{\sqrt{u}-\sqrt{v}}\)

\(=\dfrac{u-\sqrt{uv}-\sqrt{uv}+v-u+\sqrt{uv}-v}{\sqrt{u}-\sqrt{v}}\)

\(\Leftrightarrow\)\(-\dfrac{\sqrt{uv}}{\sqrt{u}-\sqrt{v}}\)

16 tháng 8 2018

mình chưa hiểu bài giải này ạ

16 tháng 8 2018

\(B=\dfrac{2u+\sqrt{uv}-3v}{2u-5\sqrt{uv}+3v}\)

\(=\dfrac{2u+3\sqrt{uv}-2\sqrt{uv}-3v}{2u-2\sqrt{uv}-3\sqrt{uv}+3v}\)

\(=\dfrac{\sqrt{u}.\left(2\sqrt{u}+3\sqrt{v}\right)-\sqrt{v}.\left(2\sqrt{u}+3\sqrt{v}\right)}{2\sqrt{u}.\left(\sqrt{u}-\sqrt{v}\right)-3\sqrt{v}.\left(\sqrt{u}-\sqrt{v}\right)}\)

\(=\dfrac{\left(2\sqrt{u}+3\sqrt{v}\right)\left(\sqrt{u}-\sqrt{v}\right)}{\left(\sqrt{u}-\sqrt{v}\right)\left(2\sqrt{u}-3\sqrt{v}\right)}\)

\(=\dfrac{2\sqrt{u}+3\sqrt{v}}{2\sqrt{u}-3\sqrt{v}}\\ =\dfrac{4u+12\sqrt{uv}+9v}{4u-9v}\)

11 tháng 8 2020

mình có sửa lại đề 1 chút!

đặt \(T=\sqrt{\frac{u-8\sqrt[6]{u^3v^2}+4\sqrt[3]{v^2}}{\sqrt{u}-2\sqrt[3]{v}+2\sqrt[12]{u^3v^2}}+3\sqrt[3]{v}}+\sqrt[6]{v}=1\)

đặt \(u=a^4;v=b^6\)(a,b>0) ta có

\(T=\frac{u-8\sqrt[6]{u^3v^2}+4\sqrt[3]{v^2}}{\sqrt{u}-2\sqrt[3]{v}+2\sqrt[12]{u^3v^2}}+3\sqrt[3]{v}=\frac{a^4-8a^2b^2+4b^2}{a^2-2b^2+2ab}+3b^2\)

vậy \(T=\frac{a^4-8a^2b^2+4b^4}{a^2-2b^2+2ab}+3b^2=\frac{a^4-5a^2b^2-2b^4+6ab^3}{a^2-2b^2+2ab}=a^2-2ab+b^2\)

từ đó suy ra \(\sqrt{\frac{u-8\sqrt[6]{u^3v^2}+4\sqrt[3]{v^2}}{\sqrt{u}-2\sqrt[3]{v}+2\sqrt[12]{u^3v^2}}+3\sqrt[3]{v}}+\sqrt[6]{v}=\left|\sqrt[4]{u}-\sqrt[6]{v}\right|+\sqrt[6]{v}\)

vì \(u^3\ge v^2\)nên \(\left|\sqrt[4]{u}-\sqrt[6]{v}\right|+\sqrt[6]{v}=\sqrt[4]{u}\)

\(\sqrt{\frac{u-8\sqrt[6]{u^3v^2}+4\sqrt[3]{v^2}}{\sqrt{u}-2\sqrt[3]{v}+2\sqrt[12]{u^3v^2}}+3\sqrt[3]{v}}+\sqrt[6]{v}=1\)

với u=1 ta có \(T=\sqrt{\frac{1-8\sqrt[6]{v^2}+4\sqrt[3]{v^2}}{1-2\sqrt[3]{v}+2\sqrt[6]{v^2}}+3\sqrt[3]{v}}+\sqrt[6]{v}\)

nếu \(1-2\sqrt[3]{v}+2\sqrt[6]{v}=0\)thì \(\sqrt[3]{v}=\frac{3+1}{2}>0\)

do \(v^2>1=u^3\), mâu thuẫn suy ra \(1-2\sqrt[3]{v}+2\sqrt[6]{v}\ne0\)

tóm lại với \(u^3\ge v^2\)và u,v\(\inℚ^+\)để \(\sqrt{\frac{u-8\sqrt[6]{u^3v^2}+4\sqrt[3]{v^2}}{\sqrt{u}-2\sqrt[3]{v}+2\sqrt[12]{u^3v^2}}+3\sqrt[3]{v}}+\sqrt[6]{v}=1\)cần và đủ là u=1 và v<1, v\(\inℚ^+\)được lấy tùy ý

NV
13 tháng 6 2019

\(M=\frac{2\sqrt{y}}{x-y}+\frac{\sqrt{x}+\sqrt{y}}{x-y}+\frac{\sqrt{x}-\sqrt{y}}{x-y}=\frac{2\sqrt{y}+\sqrt{x}+\sqrt{y}+\sqrt{x}-\sqrt{y}}{x-y}=\frac{2\left(\sqrt{x}+\sqrt{y}\right)}{\left(\sqrt{x}+\sqrt{y}\right)\left(\sqrt{x}-\sqrt{y}\right)}=\frac{2}{\sqrt{x}-\sqrt{y}}\)

b/ Khi \(x=4y\) và M=1

\(\Leftrightarrow\frac{2}{\sqrt{4y}-\sqrt{y}}=1\Leftrightarrow\frac{2}{2\sqrt{y}-\sqrt{y}}=1\Leftrightarrow\frac{2}{\sqrt{y}}=1\)

\(\Leftrightarrow\sqrt{y}=2\Rightarrow y=4\Rightarrow x=16\)

19 tháng 8 2020

Bài 1 : 

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

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

\(P=\frac{1+\sqrt{x}}{\sqrt{x}\left(\sqrt{x}-1\right)}.\frac{\sqrt{x}-1}{\sqrt{x}}\)

\(P=\frac{\sqrt{x}+1}{x}\)

b) \(P>\frac{1}{2}\)

\(\Leftrightarrow\frac{\sqrt{x}+1}{x}>\frac{1}{2}\)

\(\Leftrightarrow\frac{\sqrt{x}+1}{x}-\frac{1}{2}>0\)

\(\Leftrightarrow\frac{\sqrt{x}+1-2x}{x}>0\)

\(\Leftrightarrow\sqrt{x}-2x+1>0\left(x>0\right)\)

\(\Leftrightarrow\sqrt{x}+x^2-2x+1-x^2>0\)

\(\Leftrightarrow\sqrt{x}+x^2+\left(x-1\right)^2>0\left(\forall x>0\right)\)

Vậy P > 1/2 với mọi x> 0 ; x khác 1

19 tháng 8 2020

Bài 2 : 

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

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

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

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

\(K=\frac{\left(a-1\right)^2}{3a+2\sqrt{a}-1}\)

b) \(a=3+2\sqrt{2}=2+2\sqrt{2}+1=\left(\sqrt{2}+1\right)^2\)( thỏa mãn ĐKXĐ )

Thay a vào biểu thức K , ta có :

\(K=\frac{\left(3+2\sqrt{2}-1\right)^2}{3\left(3+2\sqrt{2}\right)+2\sqrt{\left(\sqrt{2}+1\right)^2}-1}\)

\(K=\frac{\left(2+2\sqrt{2}\right)^2}{9+6\sqrt{2}+2\left|\sqrt{2}+1\right|-1}\)

\(K=\frac{\left(2+2\sqrt{2}\right)^2}{8+6\sqrt{2}+2\sqrt{2}+2}\)

\(K=\frac{\left(2+2\sqrt{2}\right)^2}{10+8\sqrt{2}}\)

4 tháng 11 2019

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

\(=\frac{\sqrt{x}-\left(\sqrt{x}-1\right)}{\sqrt{x}\left(\sqrt{x}-1\right)}:\frac{\left(\sqrt{x}+1\right)\left(\sqrt{x}-1\right)-\left(\sqrt{x}-2\right)\left(\sqrt{x}+2\right)}{\left(\sqrt{x}-2\right)\left(\sqrt{x}-1\right)}\)\(\left(x>0,x\ne4,x\ne1\right)\)

\(=\frac{1}{\sqrt{x}\left(\sqrt{x}-1\right)}:\frac{x-1-\left(x-4\right)}{\left(\sqrt{x}-2\right)\left(\sqrt{x}-1\right)}\)

\(=\frac{1}{\sqrt{x}\left(\sqrt{x}-1\right)}:\frac{3}{\left(\sqrt{x}-2\right)\cdot\left(\sqrt{x}-1\right)}\)

\(=\frac{1}{\sqrt{x}\left(\sqrt{x}-1\right)}\cdot\frac{\left(\sqrt{x}-2\right)\left(\sqrt{x}-1\right)}{3}\)

\(=\frac{\sqrt{x}-2}{3\sqrt{x}}\)

ta có \(x=3-2\sqrt{2}\)

\(\Leftrightarrow x=2+1-2\sqrt{2}\)

\(\Leftrightarrow x=\left(\sqrt{2}-1\right)^2\)

thay x và biểu thức ta có :

\(\frac{\sqrt{\left(\sqrt{2}-1\right)^2}-2}{3\sqrt{\left(\sqrt{2}-1\right)^2}}\)

\(=\frac{\left|\sqrt{2}-1\right|-2}{3\left|\sqrt{2}-1\right|}\)

\(=\frac{\sqrt{2}-1-2}{3\left(\sqrt{2}-1\right)}\)

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

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

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

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

có sai cho mình xin lỗi nha !

4 tháng 11 2019

Giúp toi với

23 tháng 10 2020

\(A=\frac{\sqrt{x}+\sqrt{y}}{\sqrt{x}-\sqrt{y}}-\frac{\sqrt{x}-\sqrt{y}}{\sqrt{x}+\sqrt{y}}\)

ĐK : \(\hept{\begin{cases}x,y>0\\x\ne y\end{cases}}\)

\(=\frac{\left(\sqrt{x}+\sqrt{y}\right)\left(\sqrt{x}+\sqrt{y}\right)}{\left(\sqrt{x}-\sqrt{y}\right)\left(\sqrt{x}+\sqrt{y}\right)}-\frac{\left(\sqrt{x}-\sqrt{y}\right)\left(\sqrt{x}-\sqrt{y}\right)}{\left(\sqrt{x}-\sqrt{y}\right)\left(\sqrt{x}+\sqrt{y}\right)}\)

\(=\frac{x+2\sqrt{xy}+y}{x-y}-\frac{x-2\sqrt{xy}+y}{x-y}\)

\(=\frac{x+2\sqrt{xy}+y-x+2\sqrt{xy}-y}{x-y}=\frac{4\sqrt{xy}}{x-y}\)

Với \(\hept{\begin{cases}x=7+2\sqrt{3}\\y=7-2\sqrt{3}\end{cases}}\)( tmđk )

=> \(A=\frac{4\sqrt{\left(7+2\sqrt{3}\right)\left(7-2\sqrt{3}\right)}}{7+2\sqrt{3}-\left(7-2\sqrt{3}\right)}\)

\(=\frac{4\sqrt{7^2-\left(2\sqrt{3}\right)^2}}{7+2\sqrt{3}-7+2\sqrt{3}}\)

\(=\frac{4\sqrt{49-12}}{4\sqrt{3}}\)

\(=\frac{4\sqrt{37}}{4\sqrt{3}}=\frac{\sqrt{37}}{\sqrt{3}}=\frac{\sqrt{37}\cdot\sqrt{3}}{\sqrt{3}\cdot\sqrt{3}}=\frac{\sqrt{111}}{3}\)