a) Ta có:

\(\dfrac{a^2}{a-1}\) \(\geq\) 4(*)

\(\Leftrightarrow\) a² \(\geq\) 4.(a-1)

\(\Leftrightarrow\) a² \(\geq\) 4a-4

\(\Leftrightarrow\) a²-4a+4 \(\geq\) 0

\(\Leftrightarrow\) (a-2)² \(\geq\) 0(**)

Ta có BĐT(**) luôn đúng nên suy ra BĐT(*) luôn đúng

Dấu = xảy ra khi và chỉ khi a=2

B) Áp dụng câu a ta được:

\(\dfrac{4a^2}{a-1}=4.\dfrac{a^2}{a-1}\) \(\geq\) 4.4=16(1)

\(\dfrac{5b^2}{b-1}=5.\dfrac{b^2}{b-1}\) \(\geq\) 5.4=20(2)

\(\dfrac{3c^2}{c-1}=3.\dfrac{c^2}{c-1}\) \(\geq\) 3.4=12(3)

Cộng các BĐT(1),(2),(3) ta được

\(\dfrac{4a^2}{a-1}+\dfrac{5b^2}{b-1}+\dfrac{3c^2}{c-1}\) \(\geq\) 16+20+12=48

Dấu = xảy ra khi và chỉ khi a=b=c=2

Đặt A= \(\dfrac{4a^2}{a-1}+\dfrac{8b^2}{b-1}+\dfrac{12c^2}{c-1}\)

Áp dụng BĐT đã CM ta có:

A= \(\dfrac{4a^2}{a-1}+\dfrac{8b^2}{b-1}+\dfrac{12c^2}{c-1}\) \(\geq\) 4.4+8.4+12.4=16+32+48=96

\(\Rightarrow\) \(\dfrac{4a^2}{a-1}+\dfrac{8b^2}{b-1}+\dfrac{12c^2}{c-1}\) \(\geq\) 96

hay A \(\geq\) 96

Dấu = xảy ra khi và chỉ khi a=b=c=2

Vậy Min_A=96 khi và chỉ khi a=b=c=2

a)

Ta có :

\(\dfrac{a^2}{a-1}\ge4\) (1)

\(\Leftrightarrow\dfrac{a^2}{a-1}\ge\dfrac{4a-4}{a-1}\left(\forall a-1\ne0\right)\)

\(\Leftrightarrow a^2\ge4a-4\)

\(\Leftrightarrow a^2-4a+4\ge0\)

\(\Leftrightarrow\left(a-2\right)^2\ge0\)(luôn đúng) (2)

BĐT (2) đúng suy ra BĐT (1) luôn đúng

Dấu bằng xảy ra chỉ khi và khi a = 2

Đặt \(\dfrac{a}{b}=\dfrac{c}{d}=k\)

=>\(a=bk;c=dk\)

1: \(\dfrac{2a+3c}{2b+3d}=\dfrac{2\cdot bk+3\cdot dk}{2b+3d}=\dfrac{k\left(2b+3d\right)}{2b+3d}=k\)

\(\dfrac{2a-3c}{2b-3d}=\dfrac{2bk-3dk}{2b-3d}=\dfrac{k\left(2b-3d\right)}{2b-3d}=k\)

Do đó: \(\dfrac{2a+3c}{2b+3d}=\dfrac{2a-3c}{2b-3d}\)

2: \(\dfrac{4a-3b}{4c-3d}=\dfrac{4\cdot bk-3b}{4\cdot dk-3d}=\dfrac{b\left(4k-3\right)}{d\left(4k-3\right)}=\dfrac{b}{d}\)

\(\dfrac{4a+3b}{4c+3d}=\dfrac{4bk+3b}{4dk+3d}=\dfrac{b\left(4k+3\right)}{d\left(4k+3\right)}=\dfrac{b}{d}\)

Do đó: \(\dfrac{4a-3b}{4c-3d}=\dfrac{4a+3b}{4c+3d}\)

3: \(\dfrac{3a+5b}{3a-5b}=\dfrac{3bk+5b}{3bk-5b}=\dfrac{b\left(3k+5\right)}{b\left(3k-5\right)}=\dfrac{3k+5}{3k-5}\)

\(\dfrac{3c+5d}{3c-5d}=\dfrac{3dk+5d}{3dk-5d}=\dfrac{d\left(3k+5\right)}{d\left(3k-5\right)}=\dfrac{3k+5}{3k-5}\)

Do đó: \(\dfrac{3a+5b}{3a-5b}=\dfrac{3c+5d}{3c-5d}\)

4: \(\dfrac{3a-7b}{b}=\dfrac{3bk-7b}{b}=\dfrac{b\left(3k-7\right)}{b}=3k-7\)

\(\dfrac{3c-7d}{d}=\dfrac{3dk-7d}{d}=\dfrac{d\left(3k-7\right)}{d}=3k-7\)

Do đó: \(\dfrac{3a-7b}{b}=\dfrac{3c-7d}{d}\)

qua vo van

Thôi làm luôn nãy h chém nhiều mỏi tay quá. Bổ sung điều kiện a;b;c>1

\(\dfrac{4a^2}{a-1}+\dfrac{5b^2}{b-1}+\dfrac{3c^2}{c-1}\ge48\)

\(\Rightarrow\left(\dfrac{4a^2}{a-1}-16\right)+\left(\dfrac{5b^2}{b-1}-20\right)+\left(\dfrac{3c^2}{c-1}-12\right)\ge0\)

\(\Rightarrow\dfrac{4a^2-16a+16}{a-1}+\dfrac{5b^2-20b+20}{b-1}+\dfrac{3c^2-12c+12}{c-1}\ge0\)

\(\Rightarrow\dfrac{4\left(a-2\right)^2}{a-1}+\dfrac{5\left(b-2\right)^2}{b-1}+\dfrac{3\left(c-2\right)^2}{c-1}\ge0\) (đúng)

Dấu "=" khi \(a=b=c=2\)

Nhận xét :

Nhìn vào bất đẳng thức dễ thấy ở phần tử các aanrr đều ở bậc 2 còn mẫu thì lại bậc 1 nên cần điều kiện rõ ràng hơn cho a,b và c

Tử số của các phân tử luôn dương , với điều kiện a,b,c > 0 thì mẫu rõ ràng có thể nhận giá trị âm khiên cả biểu thức bé hơn không ( mâu thuẫn đề ra ). Ví dụ khi a=b=c=\(\dfrac{1}{2}\)

=> VT \(=\dfrac{1}{1-\dfrac{1}{2}}\left(4a^2+5b^2+6c^2\right)=-2\left(4a^2+5b^2+6c^2\right)< 0\)(1)

Mà VT \(\ge48\)(2)

Thấy (1) và (2) mâu thuẫn

=> Đề sai hoặc thiểu điều kiện cho a,b và c

1) Đặt \(\dfrac{a}{b}=\dfrac{c}{d}=k\Rightarrow\left\{{}\begin{matrix}a=bk\\c=dk\end{matrix}\right.\)

Ta có \(\dfrac{3a+5b}{3a-5b}=\dfrac{3bk+5b}{3bk-5b}=\dfrac{b\left(3k+5\right)}{b\left(3k-5\right)}=\dfrac{3k+5}{3k-5}\) (1)

\(\dfrac{3c+5d}{3c-5d}=\dfrac{3dk+5d}{3dk-5d}=\dfrac{d\left(3k+5\right)}{d\left(3k-5\right)}=\dfrac{3k+5}{3k-5}\) (2)

Từ (1) và (2) \(\Rightarrow\dfrac{3a+5b}{3a-5b}=\dfrac{3c+5d}{3c-5d}\)

2) Đặt \(\dfrac{a}{b}=\dfrac{c}{d}=q\Rightarrow\left\{{}\begin{matrix}a=bq\\c=dq\end{matrix}\right.\)

Ta có: \(\left(\dfrac{a+b}{c+d}\right)^2=\left(\dfrac{bq+b}{dq+d}\right)^2=\left[\dfrac{b\left(q+1\right)}{d\left(q+1\right)}\right]^2=\dfrac{b}{d}\) (1)

\(\dfrac{a^2+b^2}{c^2+d^2}=\dfrac{\left(bq\right)^2+b^2}{\left(dq\right)^2+d^2}=\dfrac{b^2.q^2+b^2}{d^2.q^2+d^2}=\dfrac{b^2\left(q^2+1\right)}{d^2\left(q^2+1\right)}=\dfrac{b^2}{d^2}=\dfrac{b}{d}\) (2)

Từ (1) và (2) \(\Rightarrow\left(\dfrac{a+b}{c+d}\right)^2=\dfrac{a^2+b^2}{c^2+d^2}\)

\(\dfrac{4}{x}-\dfrac{y}{3}=\dfrac{1}{6}\)

\(\Rightarrow\dfrac{4}{x}-\dfrac{2y}{6}=\dfrac{1}{6}\)

\(\Rightarrow\dfrac{4}{x}=\dfrac{1}{6}+\dfrac{2y}{6}\)

\(\Rightarrow\dfrac{4}{x}=\dfrac{1+2y}{6}\)

\(\Rightarrow24=x\left(1+2y\right)\)

\(\Rightarrow x;1+2y\inƯ\left(24\right)\)

\(Ư\left(24\right)=\left\{\pm1;\pm2;\pm3;\pm4;\pm6;\pm8;\pm12;\pm24\right\}\)

Mà 1+2y lẻ nên:

\(\left\{{}\begin{matrix}1+2y=1\Rightarrow2y=0\Rightarrow y=0\\x=24\\1+2y=-1\Rightarrow2y=-2\Rightarrow y=-1\\x=-24\end{matrix}\right.\)

\(\left\{{}\begin{matrix}1+2y=3\Rightarrow2y=2\Rightarrow y=1\\x=8\\1+2y=-3\Rightarrow2y=-4\Rightarrow y=-2\\x=-8\end{matrix}\right.\)

\(VT=\dfrac{a^3bc}{c+ab^2c}+\dfrac{ab^3c}{a+abc^2}+\dfrac{abc^3}{b+a^2bc}\)

\(=abc\left(\dfrac{a^2}{c+ab^2c}+\dfrac{b^2}{a+abc^2}+\dfrac{c^2}{b+a^2bc}\right)\)

Áp dụng bđt Cauchy-Schwarz dạng engel có:

\(VT\ge\dfrac{abc\left(a+b+c\right)^2}{a+b+c+abc\left(a+b+c\right)}\)\(=\dfrac{abc\left(a+b+c\right)}{1+abc}\)

Dấu "=" xảy ra khi \(a=b=c\)

Vậy...

Sai đề không bạn,tại a=b=c=2 thay vào không thỏa mãn nha

Đặt a/b=c/d=k

=>a=bk; c=dk

a: \(\dfrac{3a+5b}{3a-5b}=\dfrac{3bk+5b}{3bk-5b}=\dfrac{3k+5}{3k-5}\)

\(\dfrac{3c+5d}{3c-5d}=\dfrac{3dk+5d}{3dk-5d}=\dfrac{3k+5}{3k-5}\)

Do đó: \(\dfrac{3a+5b}{3a-5b}=\dfrac{3c+5d}{3c-5d}\)

b: \(\left(\dfrac{a+b}{c+d}\right)^2=\left(\dfrac{bk+b}{dk+d}\right)^2=\left(\dfrac{b}{d}\right)^2\)

\(\dfrac{a^2+b^2}{c^2+d^2}=\dfrac{b^2k^2+b^2}{d^2k^2+d^2}=\dfrac{b^2}{d^2}\)

Do đó: \(\left(\dfrac{a+b}{c+d}\right)^2=\dfrac{a^2+b^2}{c^2+d^2}\)

c: \(\dfrac{a-b}{a+b}=\dfrac{bk-b}{bk+b}=\dfrac{k-1}{k+1}\)

\(\dfrac{c-d}{c+d}=\dfrac{dk-d}{dk+d}=\dfrac{k-1}{k+1}\)

Do đó: \(\dfrac{a-b}{a+b}=\dfrac{c-d}{c+d}\)

Theo bất đẳng thức tam giác

\(\Rightarrow\left\{\begin{matrix}a< b+c\\b< c+a\\c< a+b\end{matrix}\right.\Rightarrow\left\{\begin{matrix}b+c-a>0\\c+a-b>0\\a+b-c>0\end{matrix}\right.\)

Áp dụng bất đẳng thức \(\dfrac{1}{a}+\dfrac{1}{b}\ge\dfrac{4}{a+b}\forall a,b>0\)

\(\Rightarrow\left\{\begin{matrix}\dfrac{1}{a+b-c}+\dfrac{1}{b+c-a}\ge\dfrac{2}{b}\\\dfrac{1}{b+c-a}+\dfrac{1}{a+c-b}\ge\dfrac{2}{c}\\\dfrac{1}{a+b-c}+\dfrac{1}{a+c-b}\ge\dfrac{2}{a}\end{matrix}\right.\)

Cộng theo từng vế

\(\Rightarrow2\left(\dfrac{1}{a+b-c}+\dfrac{1}{b+c-a}+\dfrac{1}{a+c-b}\right)\ge2\left(\dfrac{1}{a}+\dfrac{1}{b}+\dfrac{1}{c}\right)\)

\(\Rightarrow\dfrac{1}{a+b-c}+\dfrac{1}{b+c-a}+\dfrac{1}{a+c-b}\ge\dfrac{1}{a}+\dfrac{1}{b}+\dfrac{1}{c}\) ( đpcm )

câu 1: a+b>?

Câu 1: mik sửa đề tí

Ta có: a+b=1

a² +b² ≥ (a+b)²/2

<=> a² +b² ≥ 1/2(a² +b²) + ab

<=> 1/2(a² +b²) -ab ≥ 0

<=> 1/2(a-b)² ≥ 0 ( luôn đúng )

vậy a² + b² ≥ (a+b)²/2 = 1/2

tương tự thì

a^4 + b^4 ≥ (a² +b²)²/2 ≥ (1/2)²/2 = 1/8

vậy a^4 + b^4 ≥ 1/8

dấu = xảy ra <=> a=b=1/2

\(1,Q=\dfrac{a^4-2a^2+a^3-2a+a^2-2}{a^4-2a^2+2a^3-4a+a^2-2}\\ Q=\dfrac{\left(a^2-2\right)\left(a^2+a+1\right)}{\left(a^2-2\right)\left(a^2+2a+1\right)}=\dfrac{a^2+a+1}{a^2+2a+1}\)

\(Q=\dfrac{x^2+x+1}{\left(x+1\right)^2}-\dfrac{3}{4}+\dfrac{3}{4}=\dfrac{x^2+x+1-\dfrac{3}{4}x^2-\dfrac{3}{2}x-\dfrac{3}{4}}{\left(x+1\right)^2}+\dfrac{3}{4}\\ Q=\dfrac{\dfrac{1}{4}x^2-\dfrac{1}{2}x+\dfrac{1}{4}}{\left(x+1\right)^2}+\dfrac{3}{4}=\dfrac{\dfrac{1}{4}\left(x-1\right)^2}{\left(x+1\right)^2}+\dfrac{3}{4}\ge\dfrac{3}{4}\\ Q_{min}=\dfrac{3}{4}\Leftrightarrow x=1\)

\(2,\text{Từ GT }\Leftrightarrow\dfrac{ayz+bxz+czy}{xyz}=0\\ \Leftrightarrow ayz+bxz+czy=0\\ \text{Ta có }\dfrac{x}{a}+\dfrac{y}{b}+\dfrac{z}{c}=1\\ \Leftrightarrow\left(\dfrac{x}{a}+\dfrac{y}{b}+\dfrac{z}{c}\right)^2=1\\ \Leftrightarrow\dfrac{x^2}{a^2}+\dfrac{y^2}{b^2}+\dfrac{z^2}{c^2}+2\left(\dfrac{xy}{ab}+\dfrac{yz}{bc}+\dfrac{zx}{ca}\right)=0\\ \Leftrightarrow\dfrac{x^2}{a^2}+\dfrac{y^2}{b^2}+\dfrac{z^2}{c^2}+2\cdot\dfrac{cxy+ayz+bzx}{abc}=1\\ \Leftrightarrow\dfrac{x^2}{a^2}+\dfrac{y^2}{b^2}+\dfrac{z^2}{c^2}+2\cdot\dfrac{0}{abc}=1\\ \Leftrightarrow\dfrac{x^2}{a^2}+\dfrac{y^2}{b^2}+\dfrac{z^2}{c^2}=1\)