Lời giải:

Đặt $\frac{a}{b}=\frac{c}{d}=k$

$\Rightarrow a=bk, c=dk$. Khi đó:

$\frac{a-b}{b}=\frac{bk-b}{b}=\frac{b(k-1)}{b}=k-1(1)$

$\frac{c-d}{d}=\frac{dk-d}{d}=\frac{d(k-1)}{d}=k-1(2)$

Từ $(1); (2)\Rightarrow \frac{a-b}{b}=\frac{c-d}{d}$

-------------------

$\frac{2a+3b}{2a-3b}=\frac{2bk+3b}{2bk-3b}=\frac{b(2k+3)}{b(2k-3)}=\frac{2k+3}{2k-3}(3)$

$\frac{2c+3d}{2c-3d}=\frac{2dk+3d}{2dk-3d}=\frac{d(2k+3)}{d(2k-3)}=\frac{2k+3}{2k-3}(4)$

Từ $(3); (4)\Rightarrow \frac{2a+3b}{2a-3b}=\frac{2c+3d}{2c-3d}$

Lời giải:

a. Với $n$ nguyên khác -3, để $B$ nguyên thì:

$2n+9\vdots n+3$

$\Rightarrow 2(n+3)+3\vdots n+3$

$\Rightarrow 3\vdots n+3$

$\Rightarrow n+3\in\left\{\pm 1; \pm 3\right\}$

$\Rightarrow n\in\left\{-2; -4; 0; -6\right\}$

b. 

$B=\frac{2n+9}{n+3}=\frac{2(n+3)+3}{n+3}=2+\frac{3}{n+3}$

Để $B_{\max}$ thì $\frac{3}{n+3}$ max

Điều này đạt được khi $n+3$ là số nguyên dương nhỏ nhất

Tức là $n+3=1$

$\Leftrightarrow n=-2$

c. Để $B$ min thì $\frac{3}{n+3}$ min

Điều này đạt được khi $n+3$ là số nguyên âm lớn nhất 

Tức là $n+3=-1$

$\Leftrightarrow n=-4$

 x + {(x - 3) - [(x + 3) - (-x - 2)]} = x

=> x + {x - 3 - [x + 3 + x + 2]} = x

=> x + {x - 3 - x - 3 - x - 2} = x

=> x + x - 3 - x - 3 - x - 2 = x

=> (x - x) + (x - x) - (3 + 3 + 2) = x

=> 0 + 0 - 8 = x

=> - 8 = x

vậy x = - 8

=>(x-3)-[(x+3)-(-x-2)]=0

=>(x-3)-(x+3+x+2)=0

=>x-3-2x-5=0

=>-x-8=0

=>-x=8=>x=-8

Bài 2

a) x/8 = 5,4/3

x = 8 . 5,4/3

x = 14,4

b) 2,5 : 7,5 = x : 3/5

x = 3/5 × 1/3

x = 1/5

c) 2 2/3 : x = 1 7/9 : 0,2

8/3 : x = 16/9 : 1/5

x = 8/3 : (16/9 : 1/5)

x = 8/3 : 80/9

x = 3/10

d) 4/x = x/0,16

x² = 4 . 0,16

x² = 0,64

x = 0,8 hoặc x = -0,8

Bài 3

a) x/9 = y/11 và x + y = 60

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

x/9 + y/11 = (x + y)/(9 + 11) = 60/20 = 3

x/9 = 3 ⇒ x = 9.3 = 27

y/11 = 3 ⇒ y = 11.3 = 33

Vậy x = 27; y = 33

b) x/3 = y/5 ⇒ 2x/6 = y/5 và 2x - y = 8

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

2x/6 = y/5 = (2x - y)/(6 - 5) = 8/1 = 8

2x/6 = 8 ⇒ x = 6.8:2 = 24

y/5 = 8 ⇒ y = 5.8 = 40

Vậy x = 24; y = 40

c) 7x = 4y ⇒ y/7 = x/4 và y - x = 24

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

y/7 = x/4 = (y - x)/(7 - 4) = 24/3 = 8

x/4 = 8 ⇒ x = 4.8 = 32

y/7 = 8 ⇒ y = 7.8 = 56

Vậy x = 32; y = 56

Bài 4

a) x/2 = y/5 = z/7 và x - y - z = -20

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

x/2 = y/5 = z/7 = (x - y - z)/(2 - 5 - 7) = -20/(-10) = 2

x/2 = 2 ⇒ x = 2.2 = 4

y/5 = 2 ⇒ y = 5.2 = 10

z/7 = 2 z = 7.2 = 14

Vậy x = 4; y = 10; z = 14

b) x/4 = y/3 = z/9 ⇒ x/4 = y/3 = 4z/36

Và x - y + 4z = 74

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

x/4 = y/3 = 4z/36 = (x - y + 4z)/(4 - 3 + 36) = 74/37 = 2

x/4 = 2 ⇒ x = 4.2 = 8

y/3 = 2 ⇒ y = 3.2 = 6

z/9 = 2 ⇒ z = 9.2 = 18

Vậy x = 8; y = 6; z = 18

Spring is the beginning of a year. Around this time, the climate is still cold and humid. There are still some small drizzles outside but they seem not be able to stop the hundreds of colorful flowers blooming. Peach blossoms, apricot blossoms and lime trees are the most favorite plants for the Lunar New Year. Apricot blossom is yellow, peach blossom is pink, these color brighten up the sky. Spring arriving means that with New Year is coming. At this moment, everyone's heart is throbbing. The roads are filled with vehicles, everyone is happy, they go shopping to prepare for Tet holiday such as buying trees, clothes, foods.Traditionally, all of houses hang red sentences in front of the house, plant trees, wrap green cake to eat with melon onion. This is a characteris that only Vietnam has, leaves impression on the tourists. Children are especially happy to wear new clothes and receive red envelopes, are taken to visit the relatives. For me, the spring is so special, it carries so much love and happiness.
bạn thamkhaor nha

Spring has long been called the season of the festivals. Especially in some Asia countries including Vietnam, at this time there are many big and small festivals taken place all over the country. First of all, Tet is the most important event of our country. It just for a week but is well-received and well prepared. Everyone is looking forward to Tet, they go shopping for the New Year from December twenty one, prepare some items such as bonsai, candy, clothes, ..... This is a chance for them to remember their ancestors, spend time together, give each other best wishes, ... Besides Tet, there are many other festivals in the spring. Many people tend go to pagodas, temples, find the ancient land to pray for the family healthy and peaceful. Some families book themselves a short vacation, go to the famous sightseeing tours, recharge energy for a new year. Maybe spring makes people excited and happy because it has so many festivals. Thus, when talk about spring, people often think of joy, excitement.
 

the spring

Spring in Ha Noi is short and  and often from the last days of January to March or first days o April. The weather is much warmer, humid and has drizzling rains. This kind of weather is suitable for the plants to grow and flowers to blossom.  Other kinds of flowers, such as rose, peach blossom, apricot blossom, kumquat tree,…are also more beautiful than in other seasons. Tet is an important event of spring, it is an occasion to those who work far house be able to meet together in family

vì 10 có chữ số tận cùng là 0 nên ta có thể lấy 10 mũ 28+5 có thể chia hết cho 5

VD 1: a có thể chia hết cho 5 . (a.b) cũng có thể chia hết cho 5 

VD 2: 10^{28 + 5} = 10³³  có thể chia hết chi 5 vì cơ số của 10³³ có thể chia hết cho 

- Tính lại : 10³³= 10.10.10.10.10.10.10....10.10= 1000000000000000000000000000000000 mà 1000000000000000000000000000000000 có thể chia hết cho 5

a: \(=167\cdot\dfrac{67}{1000}+167\cdot\dfrac{33}{1000}=167\cdot\dfrac{1}{10}=\dfrac{167}{10}\)

a; (100 + 67) x 0,067 + (200 - 33) x 0,033

= 167 x 0,067 - 167 x 0,033

= 167 x (0,067 + 0,033)

= 167 x 0,1

= 16,7

b; 17,2 x 1,01 - 17 - 0,2

= 17,2 x 1,01 - (17 + 0,2)

= 17,2 x 1,01  - 17,2

= 17,2 x (1,01 - 1)

= 17,2 x 0,01

= 0,172

Em kiểm tra lại đề bài nhé! Tham khảo link:

 Câu hỏi của Phan Thúy Vy - Toán lớp 7 - Học toán với OnlineMath