Đặt: \(\frac{a}{b}=\frac{b}{c}=k\)

=> \(\frac{a}{b}.\frac{b}{c}=k^2\)

=> \(\frac{a}{c}=k^2\) (1)

Lại có: \(\frac{a+b}{b+c}=\frac{a}{b}=\frac{b}{c}=k\)

=> \(\left(\frac{a+b}{b+c}\right)^2=k^2\) (2)

Từ (1) và (2) => \(\left(\frac{a+b}{b+c}\right)^2=\frac{a}{c}\)

Giải:
Đặt \(\frac{a}{b}=\frac{c}{d}=k\)

\(\Rightarrow a=bk,b=ck\)

Ta có:

\(\frac{a}{c}=\frac{bk}{c}=\frac{bkk}{ck}=\frac{bkk}{b}=k^2\) (1)

\(\left(\frac{a+b}{b+c}\right)^2=\left(\frac{bk+ck}{b+c}\right)^2=\left[\frac{k\left(b+c\right)}{b+c}\right]^2=k^2\) (2)

Từ (1) và (2) suy ra \(\frac{a}{c}=\left(\frac{a+b}{b+c}\right)^2\)

Vậy \(\frac{a}{c}=\left(\frac{a+b}{b+c}\right)^2\)

\(\left(\frac{a}{c}\right)^n=\frac{a^n+b^n}{c^n+d^n}\Leftrightarrow\frac{a^n}{c^n}=\frac{a^n+b^n}{c^n+d^n}=\frac{a^n+b^n-a^n}{c^n+d^n-c^n}=\frac{b^n}{d^n}\)

\(\Leftrightarrow\left(\frac{a}{c}\right)^n=\left(\frac{b}{d}\right)^n\)

Từ đó suy ra đpcm.

Áp dụng t/c dãy tỉ số bằng nhau, ta có: \(\left(\frac{a}{c}^n\right)=\frac{a^n+b^n}{c^n+d^n}=\frac{\left(a^n+b^n\right)-a^n}{\left(c^n+d^n\right)-c^n}=\frac{b^n}{d^n}\)

=> \(\left(\frac{a}{c}\right)^n=\left(\frac{b}{d}\right)^n\Leftrightarrow\frac{a}{c}=\frac{b}{d}\Leftrightarrow\frac{a}{b}=\frac{c}{d}\)

a) Ta có: \(\frac{a+b}{a-b}=\frac{c+d}{c-d}.\)

\(\Rightarrow\left(a+b\right).\left(c-d\right)=\left(a-b\right).\left(c+d\right)\)

\(\Rightarrow ac-ad+bc-bd=ac+ad-bc-bd\)

\(\Rightarrow ac-ad+bc=ac+ad-bc\)

\(\Rightarrow ac-ad+bc-ac-ad+bc=0\)

\(\Rightarrow-2ad+2bc=0\)

\(\Rightarrow-2ad=0-2bc\)

\(\Rightarrow-2ad=-2bc\)

\(\Rightarrow2ad=2bc\)

\(\Rightarrow ad=bc\)

\(\Rightarrow\frac{a}{b}=\frac{c}{d}\left(đpcm\right).\)

b) Ta có:

Chúc bạn học tốt!

Ta có: \(\frac{a}{b}=\frac{c}{d}.\)

Áp dụng tính chất dãy tỉ số bằng nhau ta được:

\(\frac{a}{b}=\frac{c}{d}=\frac{a+c}{b+d}\) (1)

Vì \(\frac{a}{b}=\frac{c}{d}.\)

\(\Rightarrow\frac{a}{b}=\frac{c}{d}=\frac{2018a}{2018b}=\frac{2019c}{2019d}.\)

Áp dụng tính chất dãy tỉ số bằng nhau ta được:

\(\frac{a}{b}=\frac{c}{d}=\frac{2018a}{2018b}=\frac{2019c}{2019d}=\frac{2018a+2019c}{2018b+2019d}\) (2).

Từ (1) và (2) \(\Rightarrow\frac{a+c}{b+d}=\frac{2018a+2019c}{2018b+2019d}.\)

\(\Rightarrow\left(2018a+2019c\right).\left(b+d\right)=\left(a+c\right).\left(2018b+2019d\right)\left(đpcm\right).\)

Chúc bạn học tốt!

Áp dụng tính chất dãy tỉ số bằng nhau ta có :

\(\frac{2016c-a-b}{c}=\frac{2016b-a-c}{b}=\frac{2016a-b-c}{a}=\frac{2016c-a-b+2016b-a-c+2016a-b-c}{a+b+c}=\frac{2016\left(a+b+c\right)-2\left(a+b+c\right)}{a+b+c}=\frac{2014\left(a+b+c\right)}{a+b+c}=2014\)

\(\Rightarrow\left\{\begin{matrix}\frac{2016c-a-b}{c}=2014\\\frac{2016b-a-c}{b}=2014\\\frac{2016a-b-c}{a}=2014\end{matrix}\right.\)

\(\Rightarrow\left\{\begin{matrix}2016c-a-b=2014c\\2016b-a-c=2014b\\2016a-b-c=2014a\end{matrix}\right.\)

\(\Rightarrow\left\{\begin{matrix}-a-b=2014c-2016c\\-a-c=2014b-2016b\\-b-c=2014a-2016a\end{matrix}\right.\)

\(\Rightarrow\left\{\begin{matrix}-a-b=-2c\\-a-c=-2b\\-b-c=-2a\end{matrix}\right.\)

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

Ta có \(A=\left(1+\frac{a}{b}\right)\left(1+\frac{b}{c}\right)\left(1+\frac{c}{a}\right)\)

\(\Leftrightarrow A=\frac{a+b}{b}.\frac{c+b}{c}.\frac{a+c}{a}\)

Thế (1) vào biểu thức ta có :

\(A=\frac{a+b}{b}.\frac{c+b}{c}.\frac{a+c}{a}\)

\(\Rightarrow A=\frac{2c}{b}.\frac{2a}{c}.\frac{2b}{a}\)

\(\Rightarrow A=2.2.2=8\)

Vậy biểu thức A=8

Thế tui giúp hổng đc sao

Ta có: \(\frac{1}{c}=\frac{1}{2}.\left(\frac{1}{a}+\frac{1}{b}\right)\)

\(\Rightarrow\frac{1}{c}:\frac{1}{2}=\frac{1}{a}+\frac{1}{b}\)

\(\Rightarrow\frac{2}{c}=\frac{a}{1}+\frac{b}{1}\)

\(\Rightarrow\frac{2}{c}=\frac{a}{ab}+\frac{b}{ab}\)

\(\Rightarrow\frac{2}{c}=\frac{a+b}{ab}\)

\(\Rightarrow2ab=\left(a+b\right).c\)

\(\Rightarrow2ab=ac+bc\)

\(\Rightarrow ab+ab=ac+bc\)

\(\Rightarrow ab-bc=ac-ab\)

\(\Rightarrow b.\left(a-c\right)=a.\left(c-b\right)\)

\(\Rightarrow\frac{a}{b}=\frac{a-c}{c-b}\left(đpcm\right).\)

Chúc bạn học tốt!

b, \(\frac{a+b}{a+b+c}>\frac{a+b}{a+b+c+d}\); \(\frac{b+c}{b+c+a}>\frac{b+c}{a+b+c+d}\)

 \(\frac{c+d}{c+d+a}>\frac{c+d}{a+b+c+d};\frac{d+a}{a+d+b}>\frac{a+d}{a+b+c+d}\)

Cộng các bĐT trên

=> \(B>\frac{2\left(a+b+c+d\right)}{a+b+c+d}=2\)

Ta  có Với \(0< \frac{x}{y}< 1\)

=> \(\frac{x}{y}< \frac{x+z}{y+z}\)

Áp dụng ta có 

\(B>\frac{a+b+d}{a+b+c+d}+...+\frac{d+a+c}{a+b+c+d}=3\)

Vậy 2<B<3