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*Tìm min:
\(P=\dfrac{a}{1-a}+\dfrac{b}{1-b}=\dfrac{1}{1-a}-1+\dfrac{1}{1-b}-1\)
\(\ge\dfrac{4}{\left(1-a\right)+\left(1-b\right)}-2\)
\(=\dfrac{4}{2-\dfrac{1}{2}}-2=\dfrac{2}{3}\)
Dấu "=" xảy ra khi \(a=b=\dfrac{1}{4}\). Do đó minP=2/3
*Tìm max: \(a,b\ge0\)
\(P=\dfrac{a}{1-a}+\dfrac{b}{1-b}=\dfrac{a-ab+b-ab}{\left(1-a\right)\left(1-b\right)}\)
\(=\dfrac{\dfrac{1}{2}-2ab}{1-\left(a+b\right)+ab}=\dfrac{\dfrac{1}{2}-2ab}{\dfrac{1}{2}+ab}=\dfrac{\dfrac{3}{2}-2\left(\dfrac{1}{2}+ab\right)}{\dfrac{1}{2}+ab}\)
\(=\dfrac{\dfrac{3}{2}}{\dfrac{1}{2}+ab}-2\le\dfrac{\dfrac{3}{2}}{\dfrac{1}{2}}-2=1\)
Dấu "=" xảy ra khi \(\left(a;b\right)=\left(0;\dfrac{1}{2}\right),\left(\dfrac{1}{2};0\right)\)
Vậy maxP=1
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a)
\(A=\dfrac{2x^2-16x+41}{x^2-8x+22}=\dfrac{2\left(x^2-8x+22\right)-3}{x^2-8x+22}\)
\(A-2=-\dfrac{3}{x^2-8x+22}=-\dfrac{3}{\left(x-4\right)^2+6}\ge-\dfrac{3}{6}=-\dfrac{1}{2}\)
\(A\ge\dfrac{3}{2}\) khi x =4
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![](https://rs.olm.vn/images/avt/0.png?1311)
Áp dụng bất đẳng thức AM-GM ta có:
\(2P=\left(2a+2\right)\left(2b+1\right)\le\dfrac{\left(2a+2+2b+1\right)^2}{4}=\dfrac{\left[2\left(a+b\right)+3\right]^2}{4}=\dfrac{\left(2.2+3\right)^2}{4}=\dfrac{49}{4}\)\(\Rightarrow P\le\dfrac{49}{8}\)
Dấu "=" xảy ra khi \(\left\{{}\begin{matrix}a+b=2\\2a+2=2b+1\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}a=\dfrac{3}{4}\\b=\dfrac{5}{4}\end{matrix}\right.\)
Vậy \(MaxP=\dfrac{49}{8}\), đạt tại \(a=\dfrac{3}{4};b=\dfrac{5}{4}\)
Ta có: \(P=\left(a+1\right)\left(2b+1\right)=2ab+a+2b+1=2ab+b+3=b\left(2a+1\right)+3\ge0.\left(2a+1\right)+3=3\)Dấu "=" xảy ra khi \(a=2;b=0\)
Vậy \(MinP=3\), đạt tại \(a=2;b=0\)
a.
\(F=\dfrac{a}{b+2}\Rightarrow F.b+2F=a\)
\(\Rightarrow2F=a-F.b\)
\(\Rightarrow4F^2=\left(a-F.b\right)^2\le\left(a^2+b^2\right)\left(1^2+F^2\right)=F^2+1\)
\(\Rightarrow3F^2\le1\)
\(\Rightarrow-\dfrac{1}{\sqrt{3}}\le F\le\dfrac{1}{\sqrt{3}}\)
Dấu "=" lần lượt xảy ra tại \(\left(a;b\right)=\left(-\dfrac{\sqrt{3}}{2};-\dfrac{1}{2}\right)\) và \(\left(\dfrac{\sqrt{3}}{2};-\dfrac{1}{2}\right)\)
b. Đặt \(\left\{{}\begin{matrix}a+b=x\\a-2b=y\end{matrix}\right.\) quay về câu a