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a) =
=
b) =
=
=
. ( Với điều kiện b # 1)
c) \(\dfrac{a^{\dfrac{1}{3}}b^{-\dfrac{1}{3}-}a^{-\dfrac{1}{3}}b^{\dfrac{1}{3}}}{\sqrt[3]{a^2}-\sqrt[3]{b^2}}\)= =
=
( với điều kiện a#b).
d) \(\dfrac{a^{\dfrac{1}{3}}\sqrt{b}+b^{\dfrac{1}{3}}\sqrt{a}}{\sqrt[6]{a}+\sqrt[6]{b}}\) = =
=
=
a)
\(A=\dfrac{a^{\dfrac{4}{3}}\left(a^{-\dfrac{1}{3}}+a^{\dfrac{2}{3}}\right)}{a^{\dfrac{1}{4}}\left(a^{\dfrac{3}{4}}+a^{-\dfrac{1}{4}}\right)}=\dfrac{a^{\left(\dfrac{4}{3}-\dfrac{1}{3}\right)+}a^{\left(\dfrac{4}{3}+\dfrac{2}{3}\right)}}{a^{\left(\dfrac{1}{4}+\dfrac{3}{4}\right)}+a^{\left(\dfrac{1}{4}-\dfrac{1}{4}\right)}}=\dfrac{a+a^2}{a+1}=\dfrac{a\left(a+1\right)}{a+1}\)
\(a>0\Rightarrow a+1\ne0\) \(\Rightarrow A=a\)
\(\sum_{sym}\sqrt{\dfrac{a^4+b^4}{1+ab}}=\sum_{sym}\sqrt{\dfrac{2\left(a^4+b^4\right)}{2+2ab}}>=\sum_{cyc}\dfrac{a^2}{\sqrt{2+2ab}}+\sum_{cyc}\dfrac{b^2}{\sqrt{2+2ab}}\)
\(\sum_{cyc}\dfrac{a^2}{\sqrt{2+2ab}}>=\dfrac{2\left(a+b+c\right)^2}{\sum2\sqrt{2+2ab}}>=\dfrac{3}{2}\)
\(\sum_{cyc}\dfrac{b^2}{\sqrt{2+2ab}}>=\dfrac{3}{2}\)
Cộng các BĐT trên, ta được ĐPCM
Ta có:
\(\Sigma_{sym}\sqrt{\dfrac{a^4+b^4}{1+ab}}=\Sigma_{sym}\sqrt{\dfrac{2\left(a^4+b^4\right)}{2+2ab}}\ge\Sigma_{cyc}\dfrac{a^2}{\sqrt{2+2ab}}+\Sigma_{cyc}\dfrac{b^2}{\sqrt{2+2ab}}\)
Sử dụng BĐT Cauchy - Schwarz và AM - GM có:
\(\Sigma_{cyc}\dfrac{a^2}{\sqrt{2+2ab}}\ge\dfrac{2\left(a+b+c\right)^2}{\Sigma2\sqrt{2+2ab}}\ge\dfrac{2\left(a+b+c\right)^2}{ab+bc+ca+9}\ge\dfrac{3}{2}\)
Tương tự: \(\Sigma_{cyc}\dfrac{b^2}{\sqrt{2+2ab}}\ge\dfrac{3}{2}\)
Cộng 2 BĐT ta được:
\(\sqrt{\dfrac{a^4+b^4}{1+ab}}+\sqrt{\dfrac{b^4+c^4}{1+bc}}+\sqrt{\dfrac{c^4+a^4}{1+ca}}\ge3\)
Đẳng thức xảy ra khi và chỉ khi a = b = c = 1.
\(\dfrac{a}{\sqrt{a^2+15bc}}+\dfrac{b}{\sqrt{b^2+15ca}}+\dfrac{c}{\sqrt{c^2+15ab}}\ge\dfrac{3}{4}\)
\(\Leftrightarrow\dfrac{a^2}{a\sqrt{a^2+15bc}}+\dfrac{b^2}{b\sqrt{b^2+15ca}}+\dfrac{c^2}{c\sqrt{c^2+15ab}}\ge\dfrac{3}{4}\)
Áp dụng BĐT Caushy-Schwarz ta được:
\(\dfrac{a^2}{a\sqrt{a^2+15bc}}+\dfrac{b^2}{b\sqrt{b^2+15ca}}+\dfrac{c^2}{c\sqrt{c^2+15ab}}\ge\dfrac{\left(a+b+c\right)^2}{a\sqrt{a^2+15bc}+b\sqrt{b^2+15ca}+c\sqrt{c^2+15ab}}\)
Ta chứng minh rằng:
\(a\sqrt{a^2+15bc}+b\sqrt{b^2+15ca}+c\sqrt{c^2+15ab}\le\dfrac{4}{3}\left(a+b+c\right)^2\)
\(\Leftrightarrow\sqrt{a}\sqrt{a^3+15abc}+\sqrt{b}\sqrt{b^3+15abc}+\sqrt{c}\sqrt{c^3+15abc}\le\dfrac{4}{3}\left(a+b+c\right)^2\)
Áp dụng BĐT Bunhiacopxki ta được:
\(\sqrt{a}\sqrt{a^3+15abc}+\sqrt{b}\sqrt{b^3+15abc}+\sqrt{c}\sqrt{c^3+15abc}\le\sqrt{\left(a+b+c\right)\left(a^3+b^3+c^3+45abc\right)}\)Ta tiếp tục chứng minh:
\(\dfrac{16}{9}\left(a+b+c\right)^3\ge a^3+b^3+c^3+45abc\)
\(\Leftrightarrow\dfrac{16}{9}\left(a^3+b^3+c^3+3\left(a+b\right)\left(b+c\right)\left(c+a\right)\right)\ge a^3+b^3+c^3+45abc\)
Áp dụng BĐT AM-GM (Caushy) ta được:
\(\dfrac{16}{9}\left(a^3+b^3+c^3+3\left(a+b\right)\left(b+c\right)\left(c+a\right)\right)\ge\dfrac{16}{9}\left(a^3+b^3+c^3+3.2\sqrt{ab}.2.\sqrt{bc}.2.\sqrt{ca}\right)=\dfrac{16}{9}.\left(a^3+b^3+c^3+24abc\right)\)
Ta chứng minh:
\(\dfrac{16}{9}\left(a^3+b^3+c^3+24abc\right)\ge a^3+b^3+c^3+45abc\)
\(\Leftrightarrow\dfrac{16}{9}a^3+\dfrac{16}{9}b^3+\dfrac{16}{9}c^3+\dfrac{16}{9}.24abc\ge a^3+b^3+c^3+45abc\)
\(\Leftrightarrow\dfrac{7}{9}\left(a^3+b^3+c^3\right)\ge\dfrac{7}{3}abc\) (*)
Áp dụng BĐT AM-GM (Caushy) ta được:
\(\dfrac{7}{9}\left(a^3+b^3+c^3\right)\ge\dfrac{7}{9}.3\sqrt[3]{a^3b^3c^3}=\dfrac{7}{3}abc\)
\(\Rightarrow\) (*) đúng.
Vậy BĐT đã được chứng minh. Dấu "=" xảy ra khi \(a=b=c>0\).
2.
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Câu a, b thì Nguyễn Quang Duy làm đúng rồi.
c) \(a^{\dfrac{4}{3}}:\sqrt[3]{a}=a^{\dfrac{4}{3}}:a^{\dfrac{1}{3}}=a^{\dfrac{4}{3}-\dfrac{1}{3}}=a\)
d) \(\sqrt[3]{b}:b^{\dfrac{1}{6}}=b^{\dfrac{1}{3}}:b^{\dfrac{1}{6}}=b^{\dfrac{1}{3}-\dfrac{1}{6}}=b^{\dfrac{1}{6}}\)