Số hoàn chỉnh là số có tổng các ước của nó(không kể chính nó) bằng chính nó

Vd: 6;28;...

Tổng các ước của 20 không kể 20 là:

1+2+4+5+10=22>20

=>20 ko là số hoàn chỉnh

Tổng các ước của 28 không kể 28 là:

1+2+4+7+14=28=28

=>28 là số hoàn chỉnh

Tổng các ước của 45 không kể 45 là:

1+3+5+9+15=33<45

=>45 không là số hoàn chỉnh

Tổng các ước của 128 không kể 128 là:

1+2+4+8+16+32+64=127<128

=>128 không là số hoàn chỉnh

28 

Lời giải:

ĐKĐB $\Rightarrow \frac{3x-2y}{4}=\frac{2z-4x}{3}=\frac{4y-3z}{2}$

Áp dụng TCDTSBN:

$\frac{3x-2y}{4}=\frac{2z-4x}{3}=\frac{4y-3z}{2}$

$=\frac{4(3x-2y)}{16}=\frac{3(2z-4x)}{9}=\frac{2(4y-3z)}{4}$

$=\frac{4(3x-2y)+3(2z-4x)+2(4y-3z)}{16+9+4}$

$=\frac{0}{29}=0$

$\Rightarrow 3x-2y=2z-4x=4y-3z=0$

$\Rightarrow 3x=2y; 4y=3z\Rightarrow \frac{x}{2}=\frac{y}{3}=\frac{z}{4}$

Áp dụng TCDTSBN:

$\frac{x}{2}=\frac{y}{3}=\frac{z}{4}=\frac{x+y-z}{2+3-4}=\frac{-10}{1}=-10$

$\Rightarrow x=(-10).2=-20; y=3(-10)=-30; z=4(-10)=-40$

Phương trình không có dấu = thì sao giải đc bạn ? 

Đề sai kìa 

- Trích mẫu thử.

- Hòa tan từng mẫu thử vào nước.

+ Tan: Na₂O 

PT: \(Na_2O+H_2O\rightarrow2NaOH\)

+ Không tan: CuO, Al₂O₃, MgO. (1)

- Cho mẫu thử nhóm (1) pư với HCl rồi nhỏ NaOH thu được ở thí nghiệm trên vào.

+ Có tủa xanh: CuO

PT: \(CuO+2HCl\rightarrow CuCl_2+H_2O\)

\(CuCl_2+2NaOH\rightarrow2NaCl+Cu\left(OH\right)_{2\downarrow}\)

+ Có tủa keo trắng rồi tan trong NaOH dư: Al₂O₃

PT: \(Al_2O_3+6HCl\rightarrow2AlCl_3+3H_2O\)

\(2AlCl_3+6NaOH\rightarrow2Al\left(OH\right)_{3\downarrow}+6NaCl\)

\(Al\left(OH\right)_3+NaOH\rightarrow NaAlO_2+2H_2O\)

+ Có tủa trắng: MgO

PT: \(MgO+2HCl\rightarrow MgCl_2+H_2O\)

\(MgCl_2+2NaOH\rightarrow NaCl+Mg\left(OH\right)_{2\downarrow}\)

- Dán nhãn.

giả sử : x = 5k; y = 4k; z = 3k (k là N*)

ta có: \(P=\dfrac{5k+2\left(4k\right)-3\left(3k\right)}{5k-2\left(4k\right)+3\left(3k\right)}=\dfrac{5k+8k-9k}{5k-8k+9k}=\dfrac{4k}{6k}=\dfrac{4}{6}=\dfrac{2}{3}\)

vậy P = \(\dfrac{2}{3}\)

giúp mình với mình xin cảm ơn

Mình đọc chả biết rõ đề bài là như thế nào?

\(\dfrac{\dfrac{5}{7}+\dfrac{5}{9}-\dfrac{5}{11}}{\dfrac{15}{7}+\dfrac{5}{9}-\dfrac{15}{11}}=\dfrac{\dfrac{495}{693}+\dfrac{385}{693}-\dfrac{315}{693}}{\dfrac{1485}{693}+\dfrac{385}{693}-\dfrac{945}{693}}\\ =\dfrac{\dfrac{565}{693}}{\dfrac{925}{693}}=\dfrac{565}{925}=\dfrac{113}{815}\)

a, \(S_{ABC}=\dfrac{1}{2}a.a=\dfrac{a^2}{2}\)

Theo Pytago tam giac ABC vuong tai B 

\(AC=\sqrt{a^2+a^2}=\sqrt{2}a\Rightarrow AO=\dfrac{\sqrt{2}a}{2}\)

Theo Pytago tam giac SOA vuong tai O

\(SO=\sqrt{4a^2-\dfrac{2}{4}a^2}=\sqrt{\dfrac{14a^2}{4}}=\sqrt{\dfrac{7}{2}}a\)

\(V_{ABC}=\dfrac{1}{3}.\dfrac{a^2}{2}.\dfrac{\sqrt{7}}{\sqrt{2}}a=\dfrac{a^3\sqrt{7}}{6\sqrt{2}}\)

b, Ta co \(\dfrac{d\left(C;\left(SAB\right)\right)}{d\left(O;\left(SAB\right)\right)}=\dfrac{AC}{OA}=2\Rightarrow d\left(C;\left(SAB\right)\right)=2d\left(O;\left(SAB\right)\right)\)

Ke OH vuong AB, SO vuong AB, SO;OH chua (SOH) 

=> AB vuong (SOH) 

Ke OK vuong SH => OK la khoang cach 

- bn tinh not nhe 

c, ((SAB);(ABCD)) = ^SHO 

- tinh dc phan b roi ap vao tam giac SHO la ra nhe

\(n_{Al_2O_3}=\dfrac{10,2}{27.2+16.3}=0,1\left(mol\right)\)

⇒ Số phân tử Al₂O₃ = 6,022.10²³.0,1 = 6,022.10²² (phân tử)

vì góc A và góc B là 2 góc bù nhau nên

góc A + góc B = 180 độ (1)

mà góc A = 1/6B (2)

từ (1) (2) => 1/6B + B = 180

7/6B = 180

B = 154 độ

A = 180 - 154 = 26

vậy góc A = 26 độ

TASK 2
1 My father's hobby is collecting stamps
2 She often spends hours drawing pictures every Sunday
3 The Japanese live long lives because of their healthy diet
4 We give these clothes to the homeless
5 My mother started working as an English teacher in 1996
TASK 3 + TASK 5
1 We taught English to children in a primary school last summer
2 I love spending time outdoors with my family
3 Acne affects 80% of young people
4 I enjoy listening to classical music in my spare time
5 She often carves eggshells as special gifts for her family

TASK 2. For each question, complete the second sentence so that it means the same as the first one. Use the word in brackets and do not change it. Write NO MORE THAN THREE WORDS.
Example:
0. The bookshop is opposite the library. ______ a bookshop opposite the library. (THERE)
Answer: 0. There is
1. My father often collects stamps in his free time. (HOBBY)
My ______father's hobby is______________ collecting stamps.
2. It often takes her hours to draw pictures every Sunday. (SPENDS)
She often ______spends hours drawing______________ pictures every Sunday.
3. The Japanese have a healthy diet so they live long lives. (OF)
The Japanese live long lives _______because of_____________ healthy diet.
4. We donate these clothes to children who have no home. (HOMELESS)
We give these clothes _______to homeless children_____________.
5. My mother began to teach English in 1996. (WORKING)
My mother started _________working as an___________ English teacher in 1996.
 

TASK 3. Reorder the words and phrases to make meaningful sentences.
1. taught / a primary school / We / last summer / English / in / to children / .

➝ We taught English to children in a primary school last summer.
2. being outdoors / love / with / my family / I / spending time / .

➝ I love spending time being outdoors with my family.
3. young people / affects / of / Acne / 80% / .

➝ Acne affects 80% of young people.
4. classical music / I / enjoy / in / my spare time / listening to /.

➝ I enjoy listening to classical music in my spare time.
5. carves eggshells / She / often / as / for her family / special gifts / .

➝ She often carves eggshells as special gifts for her family.
 

TASK 5. Reorder the words and phrases to make meaningful sentences.
1. taught / a primary school / We / last summer / English / in / to children / .

➝ We taught English to children in a primary school last summer.
2. being outdoors / love / with / my family / I / spending time / .

➝ I love spending time being outdoors with my family.
3. young people / affects / of / Acne / 80% / .

➝ Acne affects 80% of young people.
4. classical music / I / enjoy / in / my spare time / listening to /.

➝ She often carves eggshells as special gifts for her family.
5. carves eggshells / She / often / as / for her family / special gifts / .

➝ She often carves eggshells as special gifts for her family.
 

TASK 5. Reorder the words and phrases to make meaningful sentences.
1. taught / a primary school / We / last summer / English / in / to children / .

➝ We taught English to children in a primary school last summer.
2. being outdoors / love / with / my family / I / spending time / .

➝ I love spending time being outdoors with my family.
3. young people / affects / of / Acne / 80% / .

➝ Acne affects 80% of young people.
4. classical music / I / enjoy / in / my spare time / listening to /.

➝ She often carves eggshells as special gifts for her family.
5. carves eggshells / She / often / as / for her family / special gifts / .

➝ She often carves eggshells as special gifts for her family.
 

TASK 5. Reorder the words and phrases to make meaningful sentences.
1. taught / a primary school / We / last summer / English / in / to children / .

➝ We taught English to children in a primary school last summer.
2. being outdoors / love / with / my family / I / spending time / .

➝ I love spending time being outdoors with my family.
3. young people / affects / of / Acne / 80% / .

➝ Acne affects 80% of young people.
4. classical music / I / enjoy / in / my spare time / listening to /.

➝ She often carves eggshells as special gifts for her family.
5. carves eggshells / She / often / as / for her family / special gifts / .

➝ She often carves eggshells as special gifts for her family.
 

TASK 5. Reorder the words and phrases to make meaningful sentences.
1. taught / a primary school / We / last summer / English / in / to children / .

➝ We taught English to children in a primary school last summer.
2. being outdoors / love / with / my family / I / spending time / .

➝ I love spending time being outdoors with my family.
3. young people / affects / of / Acne / 80% / .

➝ Acne affects 80% of young people.
4. classical music / I / enjoy / in / my spare time / listening to /.

➝ She often carves eggshells as special gifts for her family.
5. carves eggshells / She / often / as / for her family / special gifts / .

➝ She often carves eggshells as special gifts for her family.