A=\(\left(\frac{\sqrt{x}-2}{x-1}-\frac{\sqrt{x}-2}{x+2\sqrt{x}+1}\right)\frac{\left(1-x\right)^2}{2}\)
a. Rút gọn biểu thức A
b. Tìm giá trị lớn nhất
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1 tháng 8 2017
E mới 7 - 8 thui !!! nhưng e sẽ cố giúp
a) \(A=\frac{\left(\sqrt{x}-2\right)\left(x+2\sqrt{x}+1\right)-\left(\sqrt{x}+2\right)\left(x-1\right)}{\left(x-1\right)\left(x+2\sqrt{x}+1\right)}.\frac{1-x^2}{2}\)
\(=\frac{x\sqrt{x}-3\sqrt{x}-2-x\sqrt{x}+\sqrt{x}-2x+2}{\left(x-1\right)\left(\sqrt{x}+1\right)^2}.\frac{1-x^2}{2}\)
\(=\frac{-2\sqrt{x}-2x}{\left(x-1\right)\left(\sqrt{x}+1\right)^2}.\frac{1-x^2}{2}\)
\(=\frac{-2\sqrt{x}\left(\sqrt{x}+1\right)}{\left(x-1\right)\left(\sqrt{x}+1\right)^2}.\frac{\left(1-x\right)\left(x+1\right)}{2}\)
\(=\frac{2\left(\sqrt{x}+1\right)\left(x-1\right)\left(x+1\right)\sqrt{x}}{2\left(\sqrt{x}+1\right)\left(x-1\right)\left(\sqrt{x}+1\right)}\)
\(=\frac{\sqrt{x}\left(x+1\right)}{\sqrt{x}+1}\)
b )
ĐKXĐ : \(x\ge0\)
Vì \(\sqrt{x}+1>0\forall x\) Để \(A=\frac{\sqrt{x}\left(x+1\right)}{\sqrt{x}+1}>0\) \(\Leftrightarrow\sqrt{x}\left(x+1\right)>0\)
\(\Rightarrow\hept{\begin{cases}\sqrt{x}\ne0\\x+1>0\end{cases}\Leftrightarrow\hept{\begin{cases}x\ne0\\x>-1\end{cases}}}\) Mà theo đxxd thì \(x\ge0\) nên \(x>0\)
Vậy với \(x>0\) thì \(A>0\)
c ) Lớp 7 chưa bt làm :((
1 tháng 8 2017
E ghi rõ nèk
\(\frac{\sqrt{x}-2}{x-1}-\frac{\sqrt{x}+2}{x+2\sqrt{x}+1}\)
\(=\frac{\left(\sqrt{x}-2\right)\left(x+2\sqrt{x}+1\right)-\left(x-1\right)\left(\sqrt{x}+2\right)}{\left(x-1\right)\left(x+2\sqrt{x}+1\right)}\)
\(=\frac{\left(x\sqrt{x}+2x+\sqrt{x}-2x-4\sqrt{x}-2\right)-\left(x\sqrt{x}+2x-\sqrt{x}-2\right)}{\left(x-1\right)\left(x+2\sqrt{x}+1\right)}\)
\(=\frac{x\sqrt{x}-3\sqrt{x}-2-x\sqrt{x}-2x+\sqrt{x}-2}{\left(x-1\right)\left(x+2\sqrt{x}+1\right)}\)
14 tháng 7 2016
ĐKXĐ : \(0\le x\ne1\)
a) \(P=\left(\frac{\sqrt{x}-2}{x-1}-\frac{\sqrt{x}+2}{x+2\sqrt{x}+1}\right).\frac{\left(1-x\right)^2}{2}\)
\(=\left[\frac{\left(\sqrt{x}-2\right)\left(\sqrt{x}+1\right)}{\left(\sqrt{x}-1\right)\left(\sqrt{x}+1\right)^2}-\frac{\left(\sqrt{x}+2\right)\left(\sqrt{x}-1\right)}{\left(\sqrt{x}-1\right)\left(\sqrt{x}+1\right)^2}\right].\frac{\left(1-x\right)^2}{2}\)
\(=\frac{x-\sqrt{x}-2-x-\sqrt{x}+2}{\left(\sqrt{x}-1\right)\left(\sqrt{x}+1\right)^2}.\frac{\left(\sqrt{x}-1\right)^2\left(\sqrt{x}+1\right)^2}{2}\)
\(=\frac{-2\sqrt{x}}{\left(\sqrt{x}-1\right)\left(\sqrt{x}+1\right)^2}.\frac{\left(\sqrt{x}-1\right)^2\left(\sqrt{x}+1\right)^2}{2}\)
\(=-\sqrt{x}\left(\sqrt{x}-1\right)\)
b) \(P=\sqrt{x}\left(1-\sqrt{x}\right)\)
Để P > 0 thì \(\hept{\begin{cases}\sqrt{x}>0\\1-\sqrt{x}>0\end{cases}\Rightarrow}0< x< 1\)
c) \(P=-x+\sqrt{x}=-\left(x-2\sqrt{x}.\frac{1}{2}+\frac{1}{4}\right)+\frac{1}{4}=-\left(\sqrt{x}-\frac{1}{2}\right)^2+\frac{1}{4}\le\frac{1}{4}\)
Vậy max P = 1/4 khi x = 1/4
