Từ a:b=3:4=>\(\frac{a}{3}\) =\(\frac{b}{4}\)

Áp dụng công thức của dãy tỉ số bằng nhau có

\(\frac{a}{3}\) =\(\frac{b}{4}\) =\(\frac{a.a+b.b}{3.3+4.4}\) =\(\frac{36}{25}\)

=> a=\(\frac{36}{25}\) .3=\(\frac{108}{25}\) và b=\(\frac{36}{25}\) .4 =\(\frac{144}{25}\)

=>\(\frac{a}{b}\) =\(\frac{108}{25}\) :\(\frac{144}{25}\)=\(\frac{108}{144}\) =\(\frac{3}{4}\)

bài này không cần giải đâu bạn vì đề bài đã cho kết quả

a:b=3:4

a/b x 4 + 1/6 = 17/6

a/b x 4 = 17/6 + 1/6

a/b x 4 = 3

a/b = 3 : 4

a/b = 3/4

Vậy a:b bằng 3/4 hoặc nếu như số thập phan thì là 0,75

- Xét: a : b = 9 : 4 \(\Rightarrow\frac{a}{9}=\frac{b}{4}\)\(\Rightarrow\frac{a}{45}=\frac{b}{20}\)

       b : c = 5 : 3 \(\Rightarrow\frac{b}{5}=\frac{c}{3}\)\(\Rightarrow\frac{b}{20}=\frac{c}{12}\)    

=> \(\frac{a}{45}=\frac{b}{20}=\frac{c}{12}\)

- Đặt: \(\frac{a}{45}=\frac{b}{20}=\frac{c}{12}=k\Rightarrow\hept{\begin{cases}a=45.k\\b=20.k\\c=12.k\end{cases}}\)

-Thay a = 45.k, b = 20.k , c = 12.k vào \(\frac{a-b}{b-c}\) ;ta có: 

\(\frac{a-b}{b-c}=\frac{45.k-20.k}{20.k-12.k}=\frac{25.k}{8.k}=\frac{25}{8}\)

Vậy \(\frac{a-b}{b-c}=\frac{25}{8}\)

dat \(\frac{a}{3}\)=\(\frac{b}{4}\)=k =>a=3k va b=4k

ma \(\frac{b}{8}\)=\(\frac{c}{9}\) nen \(\frac{4k}{8}\)=\(\frac{c}{9}\)=> c=\(\frac{9k}{2}\)

theo bai ra c+a=60 =>3k+\(\frac{9k}{2}\)=60 =>\(\frac{6k+9k}{2}\)=60 =>15k=120 => k= 8

nen a=3*8=24   b=4*8=32       c=\(\frac{9\cdot8}{2}\)=36

( A - B ) / 3 = 9

A - B = 9 * 3 = 27

Từ đây lập hiệu tỉ là ra

Ta có: \(\dfrac{a}{2}=\dfrac{3}{b}\)

nên ab=6

Ta có: a:b=4

nên a=4b

Thay a=4b vào ab=6, ta được:

\(4b^2=6\)

\(\Leftrightarrow b^2=\dfrac{3}{2}\)

hay \(b\in\left\{\dfrac{\sqrt{6}}{2};-\dfrac{\sqrt{6}}{2}\right\}\)

\(\Leftrightarrow a\in\left\{2\sqrt{6};-2\sqrt{6}\right\}\)

Ta có:

\(\dfrac{a}{2}=\dfrac{3}{b}\\ \Rightarrow a.b=3.2=6\left(1\right)\)

và theo bài ra: \(a:b=4\left(2\right) \)

Lấy \(\left(1\right)\) nhân với \(\left(2\right)\) ( nhân vế theo vế ta được:

\(a.b.a:b=6.4\\ \Leftrightarrow a^2=24\\ \Leftrightarrow\left[{}\begin{matrix}a=\sqrt{24}\\a=-\sqrt{24}\end{matrix}\right.\\ \Rightarrow\left[{}\begin{matrix}b=6:\sqrt{24}\\b=6:\left(-\sqrt{24}\right)\end{matrix}\right.\\ \Leftrightarrow\left[{}\begin{matrix}b=\dfrac{\sqrt{6}}{2}\\b=-\dfrac{\sqrt{6}}{2}\end{matrix}\right.\)

    Vậy...

a)\(\dfrac{1,2}{a}=\dfrac{2}{5}\Rightarrow a=\dfrac{1,2.5}{2}=3\)

b: Ta có: \(a:b=2:\left(-3\right)\)

nên \(\dfrac{a}{2}=\dfrac{b}{-3}\)

mà a+b=25

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

\(\dfrac{a}{2}=\dfrac{b}{-3}=\dfrac{a+b}{2-3}=\dfrac{25}{-1}=-25\)

Do đó: a=-50;b=75

b)Ta có : \(a+b=25\Rightarrow a=25-b\)

Thay vào \(\dfrac{a}{b}=\dfrac{2}{-3}\) ta được:

\(\dfrac{25-b}{b}=\dfrac{2}{-3}\\ \Rightarrow-3\left(25-b\right)=2b\\ \Rightarrow-75+3b=2b\\ \Rightarrow3b-2b=75\\ \Rightarrow b=75\)

Với b=75 thay vào a=25\(-b\) ta được:

a=25\(-75\)⇒a=\(-50\)

1 What is your hobby?

2 When did you start your hobby

3 Why do you enjoy your hobby

4 How many hours a day do you spend on your hobby

5 What does Jim usually do in his free time

6 How often does he play volleyball 

7 Where does he often play volleyball

8 Who does he like to play volleyball with

I. Write questions for the underlined parts.

1. A: What is your hobby

B: My hobby is photography.

2. A:When did you started yor hobby

B: I started my hobby five years ago.

3. A:why do you enjoy your hobby

B: I enjoy my hobby because I think it’s creative.

4. A:How many hours a day fo you spend on your hobby

B: I spend two hours a day on my hobby.

5. A:What does Jim ussually do in his free time

B: Jim usually plays volleyball in his free time.

6. A:How often does he play volleyball  

B: He plays volleyball three times a week.

7. A:Where does he often play volleyball 

B: He often plays volleyball at the court near his school.

8. A:Who does he like to play with

B: He likes to play volleyball with his friends.