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\(=\dfrac{\sqrt{ab}-b-\sqrt{a}}{\sqrt{b}}\)

21 tháng 10 2016

Có: \(a+b+c+2\sqrt{abc}=1\Rightarrow\hept{\begin{cases}a+2\sqrt{abc}=1-b-c\\b+2\sqrt{abc}=1-a-c\\c+2\sqrt{abc}=1-a-b\end{cases}}\)

\(A=\sqrt{a\left(1-b\right)\left(1-c\right)}+\sqrt{b\left(1-c\right)\left(1-a\right)}+\sqrt{c\left(1-a\right)\left(1-b\right)}-\sqrt{abc}+2015\)

\(A=\sqrt{a\left(1-b-c+bc\right)}+\sqrt{b\left(1-a-c+ac\right)}+\sqrt{c\left(1-a-b+ab\right)}-\sqrt{abc}+2015\)

\(A=\sqrt{a\left(a+2\sqrt{abc}+bc\right)}+\sqrt{b\left(b+2\sqrt{abc}+ac\right)}+\sqrt{c\left(c+2\sqrt{abc}+ab\right)}-\sqrt{abc}+2015\)

\(A=\sqrt{\left(a^2+2a\sqrt{abc}+abc\right)}+\sqrt{\left(b^2+2b\sqrt{abc}+abc\right)}+\sqrt{\left(c^2+2c\sqrt{abc}+abc\right)}-\sqrt{abc}+2015\)

\(A=\sqrt{\left(a+\sqrt{abc}\right)^2}+\sqrt{\left(b+\sqrt{abc}\right)^2}+\sqrt{\left(c+\sqrt{abc}\right)^2}-\sqrt{abc}+2015\)

\(A=a+\sqrt{abc}+b+\sqrt{abc}+c+\sqrt{abc}-\sqrt{abc}+2015\)

\(A=a+b+c+2\sqrt{abc}+2015\)

\(A=1+2015=2016\)

Vậy:....

21 tháng 10 2016
k cho mình mình k lại nhe
từ giả thiết, ta có \(\dfrac{1}{xy}+\dfrac{1}{yz}+\dfrac{1}{zx}=1\) đặt \(\left(\dfrac{1}{xy};\dfrac{1}{yz};\dfrac{1}{zx}\right)=\left(a;b;c\right)\Rightarrow a+b+c=1\) =>\(\left(\dfrac{ac}{b};\dfrac{ab}{c};\dfrac{bc}{a}\right)=\left(\dfrac{1}{x^2};\dfrac{1}{y^2};\dfrac{1}{z^2}\right)\) ta có...
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từ giả thiết, ta có \(\dfrac{1}{xy}+\dfrac{1}{yz}+\dfrac{1}{zx}=1\)

đặt \(\left(\dfrac{1}{xy};\dfrac{1}{yz};\dfrac{1}{zx}\right)=\left(a;b;c\right)\Rightarrow a+b+c=1\) =>\(\left(\dfrac{ac}{b};\dfrac{ab}{c};\dfrac{bc}{a}\right)=\left(\dfrac{1}{x^2};\dfrac{1}{y^2};\dfrac{1}{z^2}\right)\)

ta có VT=\(\dfrac{1}{\sqrt{1+\dfrac{1}{x^2}}}+\dfrac{1}{\sqrt{1+\dfrac{1}{y^2}}}+\dfrac{1}{\sqrt{1+\dfrac{1}{z^1}}}=\sqrt{\dfrac{1}{1+\dfrac{ac}{b}}}+\sqrt{\dfrac{1}{1+\dfrac{ab}{c}}}+\sqrt{\dfrac{1}{1+\dfrac{bc}{a}}}\)

=\(\dfrac{1}{\sqrt{\dfrac{b+ac}{b}}}+\dfrac{1}{\sqrt{\dfrac{a+bc}{a}}}+\dfrac{1}{\sqrt{\dfrac{c+ab}{c}}}=\sqrt{\dfrac{a}{\left(a+b\right)\left(a+c\right)}}+\sqrt{\dfrac{b}{\left(b+c\right)\left(b+a\right)}}+\sqrt{\dfrac{c}{\left(c+a\right)\left(c+b\right)}}\)

\(\le\sqrt{3}\sqrt{\dfrac{ac+ab+bc+ba+ca+cb}{\left(a+b\right)\left(b+c\right)\left(c+a\right)}}=\sqrt{3}.\sqrt{\dfrac{2\left(ab+bc+ca\right)}{\left(a+b\right)\left(b+c\right)\left(c+a\right)}}\)

ta cần chứng minh \(\sqrt{\dfrac{2\left(ab+bc+ca\right)}{\left(a+b\right)\left(b+c\right)\left(c+a\right)}}\le\dfrac{3}{2}\Leftrightarrow\dfrac{2\left(ab+bc+ca\right)}{\left(a+b\right)\left(b+c\right)\left(c+a\right)}\le\dfrac{9}{4}\Leftrightarrow8\left(ab+bc+ca\right)\le9\left(a+b\right)\left(b+c\right)\left(c+a\right)\)

<=>\(8\left(a+b+c\right)\left(ab+bc+ca\right)\le9\left(a+b\right)\left(b+c\right)\left(c+a\right)\) (luôn đúng )

^_^

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