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TT
19 tháng 9 2020
a) \(ĐKXĐ:x,y\ne0;x\ne\pm y\)
Ta có : \(A=\frac{y-x}{xy}:\left[\frac{y^2}{\left(x-y\right)^2}-\frac{2x^2y}{\left(x^2-y^2\right)^2}+\frac{x^2}{y^2-x^2}\right]\)
\(=\frac{y-x}{xy}:\left[\frac{y^2.\left(x+y\right)^2}{\left(x-y\right)^2.\left(x+y\right)^2}-\frac{2x^2y}{\left(x-y\right)^2.\left(x+y\right)^2}-\frac{x^2.\left(x^2-y^2\right)}{\left(x^2-y^2\right).\left(x^2-y^2\right)}\right]\)
\(=\frac{y-x}{xy}:\left[\frac{y^2.\left(x^2+2xy+y^2\right)-2x^2y-x^2.\left(x^2-y^2\right)}{\left(x-y\right)^2.\left(x+y\right)^2}\right]\)
\(=\frac{y-x}{xy}:\left[\frac{x^2y^2+y^4+2xy^3-2x^2y-x^4+x^2y^2}{\left(x-y\right)^2\left(x+y\right)^2}\right]\)
Đề này lỗi mình nghĩ vậy vì trên tử kia không đẹp lắm.....
30 tháng 8
Bài 3:
a: \(A=\frac{1}{2+\sqrt{x}}+\frac{1}{2-\sqrt{x}}+\frac{2\sqrt{x}}{4-x}\)
\(=\frac{1}{\sqrt{x}+2}-\frac{1}{\sqrt{x}-2}-\frac{2\sqrt{x}}{\left(\sqrt{x}+2\right)\left(\sqrt{x}-2\right)}\)
\(=\frac{\sqrt{x}-2-\sqrt{x}-2-2\sqrt{x}}{\left(\sqrt{x}+2\right)\left(\sqrt{x}-2\right)}=\frac{-2\sqrt{x}-4}{\left(\sqrt{x}+2\right)\left(\sqrt{x}-2\right)}=-\frac{2\left(\sqrt{x}+2\right)}{\left(\sqrt{x}+2\right)\left(\sqrt{x}-2\right)}\)
\(=-\frac{2}{\sqrt{x}-2}\)
b: Thay x=3 vào A, ta được: \(A=-\frac{2}{\sqrt3-2}=\frac{2}{2-\sqrt3}=2\left(2+\sqrt3\right)=4+2\sqrt3\)
Bài 2:
a: \(A=\frac{a+4\sqrt{a}+4}{\sqrt{a}+2}+\frac{4-a}{\sqrt{a}-2}\)
\(=\frac{\left(\sqrt{a}+2\right)^2}{\sqrt{a}+2}-\frac{\left(\sqrt{a}-2\right)\left(\sqrt{a}+2\right)}{\sqrt{a}-2}\)
\(=\left(\sqrt{a}+2\right)-\left(\sqrt{a}+2\right)=0\)
b: \(B=\frac{x\sqrt{x}+y\sqrt{y}}{\sqrt{x}+\sqrt{y}}:\left(\sqrt{x}-\sqrt{y}\right)^2\)
\(=\frac{\left(\sqrt{x}+\sqrt{y}\right)\left(x-\sqrt{xy}+y\right)}{\sqrt{x}+\sqrt{y}}\cdot\frac{1}{\left(\sqrt{x}-\sqrt{y}\right)^2}=\frac{x-\sqrt{xy}+y}{\left(\sqrt{x}-\sqrt{y}\right)^2}\)
Bài 1:
a: \(A=\sqrt{\frac{x-2\sqrt{x}+1}{x+2\sqrt{x}+1}}\)
\(=\sqrt{\frac{\left(\sqrt{x}-1\right)^2}{\left(\sqrt{x}+1\right)^2}}=\left|\frac{\sqrt{x}-1}{\sqrt{x}+1}\right|=\frac{\left|\sqrt{x}-1\right|}{\sqrt{x}+1}\)
b: \(B=\frac{x-1}{\sqrt{y}-1}\cdot\sqrt{\frac{\left(y-2\sqrt{y}+1\right)^2}{\left(x-1\right)^4}}\)
\(=\frac{\left(x-1\right)}{\sqrt{y}-1}\cdot\frac{\left|y-2\sqrt{y}+1\right|}{\left|\left(x-1\right)^2\right|}\)
\(=\left(x-1\right)\cdot\frac{\left(\sqrt{y}-1\right)^2}{\left(\sqrt{y}-1\right)\left(x-1\right)^2}=\frac{\left(\sqrt{y}-1\right)}{x-1}\)
30 tháng 8
Bài 2:
a: \(A=\frac{a+4\sqrt{a}+4}{\sqrt{a}+2}+\frac{4-a}{\sqrt{a}-2}\)
\(=\frac{\left(\sqrt{a}+2\right)^2}{\sqrt{a}+2}-\frac{\left(\sqrt{a}-2\right)\left(\sqrt{a}+2\right)}{\sqrt{a}-2}\)
\(=\left(\sqrt{a}+2\right)-\left(\sqrt{a}+2\right)=0\)
b: \(B=\frac{x\sqrt{x}+y\sqrt{y}}{\sqrt{x}+\sqrt{y}}:\left(\sqrt{x}-\sqrt{y}\right)^2\)
\(=\frac{\left(\sqrt{x}+\sqrt{y}\right)\left(x-\sqrt{xy}+y\right)}{\sqrt{x}+\sqrt{y}}\cdot\frac{1}{\left(\sqrt{x}-\sqrt{y}\right)^2}=\frac{x-\sqrt{xy}+y}{\left(\sqrt{x}-\sqrt{y}\right)^2}\)
Bài 1:
a: \(A=\sqrt{\frac{x-2\sqrt{x}+1}{x+2\sqrt{x}+1}}\)
\(=\sqrt{\frac{\left(\sqrt{x}-1\right)^2}{\left(\sqrt{x}+1\right)^2}}=\left|\frac{\sqrt{x}-1}{\sqrt{x}+1}\right|=\frac{\left|\sqrt{x}-1\right|}{\sqrt{x}+1}\)
b: \(B=\frac{x-1}{\sqrt{y}-1}\cdot\sqrt{\frac{\left(y-2\sqrt{y}+1\right)^2}{\left(x-1\right)^4}}\)
\(=\frac{\left(x-1\right)}{\sqrt{y}-1}\cdot\frac{\left|y-2\sqrt{y}+1\right|}{\left|\left(x-1\right)^2\right|}\)
\(=\left(x-1\right)\cdot\frac{\left(\sqrt{y}-1\right)^2}{\left(\sqrt{y}-1\right)\left(x-1\right)^2}=\frac{\left(\sqrt{y}-1\right)}{x-1}\)
26 tháng 7 2015
a) A = B : C = \(\left[\left(\frac{1}{\sqrt{x}}+\frac{1}{\sqrt{y}}\right).\frac{2}{\sqrt{x}+\sqrt{y}}+\frac{1}{x}+\frac{1}{y}\right]\). \(\frac{\sqrt{x^3y}+\sqrt{xy^3}}{\sqrt{x^3}+y\sqrt{x}+x\sqrt{y}+\sqrt{y^3}}\)
A xác định <=> x > 0 và y > 0
\(B=\left[\frac{\sqrt{x}+\sqrt{y}}{\sqrt{xy}}.\frac{2}{\sqrt{x}+\sqrt{y}}+\frac{1}{x}+\frac{1}{y}\right]=\frac{2}{\sqrt{xy}}+\frac{1}{x}+\frac{1}{y}=\left(\frac{1}{\sqrt{x}}+\frac{1}{\sqrt{y}}\right)^2\)
\(C=\frac{\sqrt{x}.\left(x+y\right)+\sqrt{y}.\left(x+y\right)}{\sqrt{xy}.\left(x+y\right)}=\frac{\left(\sqrt{x}+\sqrt{y}\right).\left(x+y\right)}{\sqrt{xy}.\left(x+y\right)}=\frac{\sqrt{x}+\sqrt{y}}{\sqrt{xy}}=\frac{1}{\sqrt{y}}+\frac{1}{\sqrt{x}}\)
=> A = B : C = \(\left(\frac{1}{\sqrt{x}}+\frac{1}{\sqrt{y}}\right)^2\) : \(\left(\frac{1}{\sqrt{y}}+\frac{1}{\sqrt{x}}\right)\) = \(\frac{1}{\sqrt{y}}+\frac{1}{\sqrt{x}}\)
c) \(A=\frac{1}{\sqrt{y}}+\frac{1}{\sqrt{x}}\ge2.\sqrt{\frac{1}{\sqrt{y}}.\frac{1}{\sqrt{x}}}=2.\sqrt{\frac{1}{\sqrt{6}}}\)
=> A nhỏ nhất bằng \(2.\sqrt{\frac{1}{\sqrt{6}}}\) khi \(\frac{1}{\sqrt{y}}=\frac{1}{\sqrt{x}}\) => x = y = \(\sqrt{6}\)
3 tháng 1 2020
\(B=\left(1-\frac{1}{x^2}\right)\left(1-\frac{1}{y^2}\right)=1-\left(\frac{1}{x^2}+\frac{1}{y^2}-\frac{1}{x^2y^2}\right)=1-\frac{x^2+y^2-1}{x^2y^2}\)
\(B=1-\frac{\left(x+y\right)^2-2xy-1}{x^2y^2}=1-\frac{-2xy}{x^2y^2}=1+\frac{2}{xy}\)
Cô-si : \(1=x+y\ge2\sqrt{xy}\Leftrightarrow xy\le\frac{1}{4}\)
\(\Rightarrow B\ge1+\frac{2}{\frac{1}{4}}=9\)
Vậy B có GTNN bằng 9 khi x = y = \(\frac{1}{2}\)
Chọn đáp án C.