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Câu hỏi của Phạm Trần Minh Trí - Toán lớp 9 - Học toán với OnlineMath
Em tham khảo.
![](https://rs.olm.vn/images/avt/0.png?1311)
Áp dụng BĐT AM-GM: \(\frac{a^3}{\left(b+c\right)^2}+\frac{b+c}{8}+\frac{b+c}{8}\ge\frac{3}{4}a\)
Suy ra \(\frac{a^3}{\left(b+c\right)^2}\ge\frac{3a-b-c}{4}\)
Tương tự các BĐT còn lại và cộng theo vế ta được \(VT\ge\frac{a+b+c}{4}=\frac{3}{2}\)
Đẳng thức xảy ra khi a = b= c = 2
![](https://rs.olm.vn/images/avt/0.png?1311)
Áp dụng BĐT AM-GM ta có:
\(6=2\left(\frac{a}{b}+\frac{b}{a}\right)+c\left(\frac{a}{b^2}+\frac{b}{a^2}\right)\)
\(\ge4+\frac{c\left(a^3+b^3\right)}{a^2b^2}\ge4+\frac{c\left(a+b\right)}{ab}\)\(\Rightarrow\frac{c\left(a+b\right)}{ab}\in\text{(}0;2\text{]}\)
Áp dụng BĐT Cauchy-Schwarz lại có:
\(P\ge\frac{\left(bc+ca\right)^2}{2abc\left(a+b+c\right)}+\frac{4}{\frac{c\left(a+b\right)}{ab}}\)\(\ge\frac{3c^2\left(a+b\right)^2}{2\left(ab+bc+ca\right)}+\frac{4}{\frac{c\left(a+b\right)}{ab}}\)
\(=\frac{\frac{3c^2\left(a+b\right)^2}{a^2b^2}}{2\left(1+\frac{ca}{ab}+\frac{bc}{ab}\right)^2}+\frac{4}{\frac{c\left(a+b\right)}{ab}}\)
\(=\frac{\frac{3c^2\left(a+b\right)^2}{a^2b^2}}{2\left[1+\frac{c\left(a+b\right)}{ab}\right]^2}+\frac{4}{\frac{c\left(a+b\right)}{ab}}\)
Đặt \(x=\frac{c\left(a+b\right)}{ab}\left(x\in\text{(}0;2\text{]}\right)\) khi đó ta có:
\(P\ge\frac{3x^2}{2\left(1+x\right)^2}+\frac{4}{x}\) cần chứng minh \(P\ge\frac{8}{3}\Leftrightarrow\left(x-2\right)\left(7x^2+22x+12\right)\le0\forall x\in\text{(0;2]}\)
Vậy \(Min_P=\frac{8}{3}\) khi a=b=c=2
![](https://rs.olm.vn/images/avt/0.png?1311)
\(\frac{9}{2\left(ab+bc+ca\right)}+\frac{2}{a^2+b^2+c^2}\)
\(=\frac{1}{2\left(ab+bc+ca\right)}+2.\left(\frac{4}{2\left(ab+bc+ca\right)}+\frac{1}{a^2+b^2+c^2}\right)\)
\(\ge\frac{1}{2.\frac{\left(a+b+c\right)^2}{3}}+2.\frac{\left(2+1\right)^2}{a^2+b^2+c^2+2\left(ab+bc+ca\right)}\)
\(=\frac{1}{2.\frac{1}{3}}+2.\frac{9}{1}=\frac{39}{2}\)
Dấu = xảy ra khi \(a=b=c=\frac{1}{3}\)
![](https://rs.olm.vn/images/avt/0.png?1311)
\(P=\left(5a+\frac{2}{b+c}\right)^2+\left(5b+\frac{2}{c+a}\right)^2+\left(5c+\frac{2}{a+b}\right)^2\)
\(=4\text{∑}\frac{1}{\left(a+b\right)^2}+20\text{ }\text{∑}\left(\frac{a}{b+c}\right)+75\)
\(\ge2\text{∑}\frac{1}{a^2+b^2}+20\cdot\frac{3}{2}+75\)
\(\ge2\cdot\frac{9}{2\left(a^2+b^2+c^2\right)}+105=108\)
Dấu = khi a=b=c=1
![](https://rs.olm.vn/images/avt/0.png?1311)
\(abc\ge\left(a+b-c\right)\left(b+c-a\right)\left(c+a-b\right)\)
\(\Leftrightarrow abc\ge\left(3-2a\right)\left(3-2b\right)\left(3-2c\right)\)
\(\Leftrightarrow9abc\ge12\left(ab+bc+ca\right)-27\)
\(\Rightarrow abc\ge\dfrac{4}{3}\left(ab+bc+ca\right)-3\)
\(P\ge\dfrac{9}{a\left(b^2+bc+c^2\right)+b\left(c^2+ca+a^2\right)+c\left(a^2+ab+b^2\right)}+\dfrac{abc}{ab+bc+ca}=\dfrac{9}{\left(ab+bc+ca\right)\left(a+b+c\right)}+\dfrac{abc}{ab+bc+ca}\)
\(\Rightarrow P\ge\dfrac{3}{ab+bc+ca}+\dfrac{abc}{ab+bc+ca}=\dfrac{3+abc}{ab+bc+ca}\)
\(\Rightarrow P\ge\dfrac{3+\dfrac{4}{3}\left(ab+bc+ca\right)-3}{ab+bc+ca}=\dfrac{4}{3}\)
Dấu "=" xảy ra khi \(a=b=c=1\)
Dat \(\left(\frac{1}{a};\frac{1}{b};\frac{1}{c}\right)=\left(x,y,z\right)\)
thi \(P= \Sigma \frac{z^2}{x+y} \geq \frac{x+y+z}{2} \) (1)
Mat khac co \(x+y+z=\frac{1}{a}+\frac{1}{b}+\frac{1}{c}\ge\frac{9}{a+b+c}=3\) (2)
Tu (1) va (2) suy ra \(P\ge\frac{3}{2}\).Dau = xay ra khi \(a=b=c=1\)