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3/ Áp dụng bất đẳng thức AM-GM, ta có :
\(\dfrac{a^2}{b^2}+\dfrac{b^2}{c^2}\ge2\sqrt{\dfrac{\left(ab\right)^2}{\left(bc\right)^2}}=\dfrac{2a}{c}\)
\(\dfrac{b^2}{c^2}+\dfrac{c^2}{a^2}\ge2\sqrt{\dfrac{\left(bc\right)^2}{\left(ac\right)^2}}=\dfrac{2b}{a}\)
\(\dfrac{c^2}{a^2}+\dfrac{a^2}{b^2}\ge2\sqrt{\dfrac{\left(ac\right)^2}{\left(ab\right)^2}}=\dfrac{2c}{b}\)
Cộng 3 vế của BĐT trên ta có :
\(2\left(\dfrac{a^2}{b^2}+\dfrac{b^2}{c^2}+\dfrac{c^2}{a^2}\right)\ge2\left(\dfrac{a}{b}+\dfrac{b}{c}+\dfrac{c}{a}\right)\)
\(\Leftrightarrow\dfrac{a^2}{b^2}+\dfrac{b^2}{c^2}+\dfrac{c^2}{a^2}\ge\dfrac{a}{b}+\dfrac{b}{c}+\dfrac{c}{a}\left(\text{đpcm}\right)\)
Bài 1:
Áp dụng BĐT AM-GM ta có:
\(\frac{1}{a^2+bc}+\frac{1}{b^2+ac}+\frac{1}{c^2+ab}\leq \frac{1}{2\sqrt{a^2.bc}}+\frac{1}{2\sqrt{b^2.ac}}+\frac{1}{2\sqrt{c^2.ab}}=\frac{\sqrt{ab}+\sqrt{bc}+\sqrt{ac}}{2abc}\)
Tiếp tục áp dụng BĐT AM-GM:
\(\sqrt{bc}+\sqrt{ac}+\sqrt{ab}\leq \frac{b+c}{2}+\frac{c+a}{2}+\frac{a+b}{2}=a+b+c\)
Do đó:
\(\frac{1}{a^2+bc}+\frac{1}{b^2+ac}+\frac{1}{c^2+ab}\leq \frac{\sqrt{ab}+\sqrt{bc}+\sqrt{ca}}{2abc}\leq \frac{a+b+c}{2abc}\) (đpcm)
Dấu "=" xảy ra khi $a=b=c$
Ta có:
\(\dfrac{a}{b}+\dfrac{b}{c}+\dfrac{c}{a}+1=\dfrac{a^2}{ab}+\dfrac{b^2}{bc}+\dfrac{c^2}{ca}+\dfrac{b^2}{b^2}\)
\(\ge\dfrac{\left(a+2b+c\right)^2}{ab+bc+ca+b^2}=\dfrac{\left(a+b\right)^2+2\left(a+b\right)\left(b+c\right)+\left(b+c\right)^2}{\left(a+b\right)\left(b+c\right)}\)
\(=\dfrac{a+b}{b+c}+\dfrac{b+c}{a+b}+2\)
Sorry bác Neet tới đây e bí mất 
Ta có: \(\dfrac{a}{b}+\dfrac{b}{c}+\dfrac{c}{a}\ge3.\sqrt[3]{\dfrac{a}{b}.\dfrac{b}{c}.\dfrac{c}{a}}=3\)(1)
\(\dfrac{a+b}{b+c}+\dfrac{b+c}{a+b}\ge2.\sqrt{\dfrac{a+b}{b+c}.\dfrac{b+c}{a+b}}=2\)
\(\Leftrightarrow\dfrac{a+b}{b+c}+\dfrac{b+c}{a+b}+1\ge3\)(2)
Từ (1), (2), ta có: \(\dfrac{a}{b}+\dfrac{b}{c}+\dfrac{c}{a}-\dfrac{a+b}{b+c}-\dfrac{b+c}{a+b}-1\ge0\)
\(\Leftrightarrow\dfrac{a}{b}+\dfrac{b}{c}+\dfrac{c}{a}\ge\dfrac{a+b}{b+c}+\dfrac{b+c}{a+b}+1\)
Dấu "=" xảy ra khi \(a=b=c\)
Giờ mới rảnh sorry :(
Theo BĐT Cauchy-Schwarz (Bunhia hay B.C.S hay Schwarz hay Cauchy....)
\(\left(ab+bc+ca\right)\left(\dfrac{a^5}{b^3}+\dfrac{b^5}{c^3}+\dfrac{c^5}{a^3}\right)\ge\left(\dfrac{a^3}{b}+\dfrac{b^3}{c}+\dfrac{c^3}{a}\right)^2\)
Cần chỉ ra \(\dfrac{a^3}{b}+\dfrac{b^3}{c}+\dfrac{c^3}{a}\ge ab+bc+ca\left(1\right)\)
Tiếp tục dùng C-S dạng Engel (hoặc Schwarz hay C-S dạng phân thức hay Svasc...)
\(VT_{\left(1\right)}=\dfrac{a^4}{ab}+\dfrac{b^4}{bc}+\dfrac{c^4}{ca}\ge\dfrac{\left(a^2+b^2+c^2\right)^2}{ab+bc+ca}\ge ab+bc+ca=VP_{\left(1\right)}\)
BĐT trên đúng nên ta có ĐPCM
\("=" \Leftrightarrow a=b=c\)
áp dụng cô si ta có :
\(\dfrac{1}{2a+b}+\dfrac{1}{2b+c}+\dfrac{1}{2c+a}\ge\dfrac{\left(1+1+1\right)^2}{2a+b+2b+c+2c+a}\)
\(=\dfrac{9}{3\left(a+b+c\right)}=\dfrac{3}{a+b+c}\)
Áp dụng BĐT Cô si Ta có : \(\dfrac{a}{b^2+1}=a-\dfrac{ab^2}{b^2+1}\ge a-\dfrac{ab^2}{2b}=a-\dfrac{ab}{2}\)
\(\dfrac{b}{c^2+1}=b-\dfrac{c^2b}{c^2+1}\ge b-\dfrac{c^2b}{2c}=b-\dfrac{cb}{2}\)
\(\dfrac{c}{a^2+1}=c-\dfrac{a^2c}{a^2+1}\ge c-\dfrac{a^2c}{2a}=c-\dfrac{ac}{2}\)
Cộng ba vế BĐT lại ta được:
\(\dfrac{a}{b^2+1}+\dfrac{b}{c^2+1}+\dfrac{c}{a^2+1}\ge a+b+c-\left(\dfrac{ab+bc+ac}{2}\right)\)
Ta có đánh giá quen thuộc \(ab+bc+ac\le\dfrac{\left(a+b+c\right)^2}{3}=\dfrac{9}{3}=3\)
\(\Rightarrow\dfrac{a}{b^2+1}+\dfrac{b}{c^2+1}+\dfrac{c}{a^2+1}\ge3-\dfrac{3}{2}=\dfrac{3}{2}\)(ĐPCM)
Đặt \(\left(x^3;y^3;z^3\right)=\left(a;b;c\right)\left(x,y,z>0\right)\)
\(\Rightarrow xyz=1\)
Ta cần chứng minh
\(\dfrac{1}{x^3+y^3+1}+\dfrac{1}{y^3+z^3+1}+\dfrac{1}{z^3+x^3+1}\le1\)
Áp dụng AM-GM, ta có: \(x^3+y^3+1=\left(x+y\right)\left(x^2-xy+y^2\right)+xyz\)
\(\ge\left(x+y\right)xy+xyz=xy\left(x+y+z\right)\)
\(\Rightarrow\dfrac{1}{x^3+y^3+1}\le\dfrac{1}{xy\left(x+y+z\right)}\)
Tương tự: \(\dfrac{1}{y^3+z^3+1}\le\dfrac{1}{yz\left(x+y+z\right)}\)
\(\dfrac{1}{z^3+x^3+1}\le\dfrac{1}{zx\left(x+y+z\right)}\)
Cộng vế theo vế, ta được
\(....\le\dfrac{1}{x+y+z}\left(\dfrac{1}{xy}+\dfrac{1}{yz}+\dfrac{1}{xz}\right)=\dfrac{1}{x+y+z}.\dfrac{x+y+z}{xyz}=\dfrac{1}{xyz}=1\)
Vậy ta có đpcm
Đẳng thức xảy ra khi a=b=c=1
Lời giải:
Áp dụng BĐT AM-GM cho các số dương:
\(a^2+bc\geq 2\sqrt{a^2bc}; b^2+ac\geq 2\sqrt{b^2ac}; c^2+ab\geq 2\sqrt{c^2ab}\)
Do đó:
\(\text{VT}=\frac{1}{a^2+bc}+\frac{1}{b^2+ac}+\frac{1}{c^2+ab}\leq \frac{1}{2\sqrt{a^2bc}}+\frac{1}{2\sqrt{b^2ac}}+\frac{1}{2\sqrt{c^2ab}}\)
hay \(\text{VT}\leq \frac{\sqrt{bc}+\sqrt{ac}+\sqrt{ab}}{2abc}(*)\)
Tiếp tục áp dụng BĐT AM-GM:
\(\left\{\begin{matrix} \sqrt{bc}\leq \frac{b+c}{2}\\ \sqrt{ac}\leq \frac{a+c}{2}\\ \sqrt{ab}\leq \frac{a+b}{2}\end{matrix}\right.\Rightarrow \sqrt{ab}+\sqrt{bc}+\sqrt{ac}\leq a+b+c(**)\)
Từ \((*);(**)\Rightarrow \text{VT}\leq \frac{a+b+c}{2abc}\)
Ta có đpcm
Dấu bằng xảy ra khi \(a=b=c\)
\(|\frac{a-b}{a+b}+\frac{b-c}{b+c}+\frac{c-a}{c+a}|=|\frac{\left(a-b\right)\left(c-a\right)\left(c-b\right)}{\left(a+b\right)\left(b+c\right)\left(c+a\right)}|\)
Không mất tính tổng quát ta giả sử \(a\ge b\ge c\)
\(\Rightarrow|\frac{\left(a-b\right)\left(c-a\right)\left(c-b\right)}{\left(a+b\right)\left(b+c\right)\left(c+a\right)}|=\frac{a-b}{a+b}.\frac{a-c}{a+c}.\frac{b-c}{b+c}< 1\)
Vì \(\left\{\begin{matrix}a-b< a+b\\b-c< b+c\\a-c< a+c\end{matrix}\right.\)
Vậy ta có ĐPCM
thanks sư phụ