I think ... school is fantastic in order to learn English and make friends ... A very nice place to study, engage in extra class activities and make friends all over the world!

Lời giải:
a.

$a(b+c)-b(a-c)=ab+ac-(ab-bc)=ab+ac-ab+bc=ac+bc$

b.

$a(b-c)-a(b+d)=ab-ac-(ab+ad)=ab-ac-ab-ad=-ac-ad$

1. A. Order B. Receive C. Complain D. Enjoy

A

Áp dụng tính chất dãy tỉ số bằng  nhau ta có

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

\(\Rightarrow\orbr{\begin{cases}a+b+c+d=0\\a=c\end{cases}}\)

Sửa đề:

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

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

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

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

Áp dụng t/c dãy tỉ số bằng nhau ta có:

\(\frac{a+b+c+d}{c+d}=\frac{c+d+b+a}{d+a}=\frac{\left(a+b+c+d\right)-\left(c+d+b+c\right)}{\left(c+d\right)-\left(d+a\right)}=\frac{0}{\left(c+d\right)-\left(d+a\right)}=0\)

\(\Rightarrow\frac{a+b+c+d}{c+d}=0\)

Vì \(c+d\ne0\)

\(\Rightarrow a+b+c+d=0\left(đpcm\right)\)

và \(\frac{a+b+c+d}{c+d}-\frac{c+d+b+a}{d+a}=0\)

vd Thay a + b+ c= 1

ta có: \(\frac{1}{c+d}-\frac{1}{d+a}=0\)

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

\(\Rightarrow d+a=c+d\)

\(\Rightarrow a=c\left(đpcm\right)\)

hok tốt!!

thank you 2b nha!!!

a) Ta có: a = -1/8 = -9/72

b = 2/-9 = -2/9 = -16/72

Ta thấy: -9 > -16 => -9/72 > -16/72

hay a > b

Vậy a > b

b) Ta có: a = 12/15 = 4/5= 16/20

b = -( -3/4 ) = 3/4= 15/20

Ta thấy: 16 > 15 => 16/20 > 15/20

hay a > b

Vậy a > b

c) Ta có: a = -2/3 = -40/60

b = -0,65 = -13/20 = -39/60

Ta thấy: -40 < -39 => -40/60 < -39/60

hay a < b

Vậy a < b

d) Ta có:  a = -21/3 = -7

b = -413% = -4,13

Ta thấy: -7 < -4,13

=> a < b

Vậy a < b

Chuk bn hok tốt! 

. 1. A. Cannon B. Chaos C. Culture D. direct

2. A. Easter B. Festive C. Rename D. harvest

3. A. Highlight B. Enjoy C. Reverse D. review

4. A. Invest B. Major C. Perform D. machine

5. A. Leather B. Account C. Adapt D. decide 

6. A. Answer B. Believe. C.Rely D.Decrease

1. A. Cannon            B. Chaos                C. Culture          D. direct

2. A. Easter              B. Festive               C. Rename        D. harvest

3. A. Highlight        B. Enjoy                  C. Reverse         D. review

4. A. Invest              B. Major                  C. Perform         D. machine

5. A. Leather            B. Account            C. Adapt            D. decide 

6. A. Answer            B. Believe.             C.Rely D.Decrease

1d

2c

3a

4b

5a

6a