a/ \(M=x^2+y^2-x+6y+10=\left(x^2-x+\frac{1}{4}\right)+\left(y^2+6y+9\right)+10-\frac{1}{4}-9\)

\(=\left(x-\frac{1}{2}\right)^2+\left(y+3\right)^2+\frac{3}{4}\ge\frac{3}{4}\)

Suy ra Min M = 3/4 <=> (x;y) = (1/2;-3)

b/

1/ \(A=4x-x^2+3=-\left(x^2-4x+4\right)+7=-\left(x-2\right)^2+7\le7\)

Suy ra Min A = 7 <=> x = 2

2/ \(B=x-x^2=-\left(x^2-x+\frac{1}{4}\right)+\frac{1}{4}=-\left(x-\frac{1}{2}\right)^2+\frac{1}{4}\le\frac{1}{4}\)

Suy ra Min B = 1/4 <=> x = 1/2

3/ \(N=2x-2x^2-5=-2\left(x^2-x+\frac{1}{4}\right)-5+\frac{1}{2}=-2\left(x-\frac{1}{2}\right)^2-\frac{9}{2}\)

\(\ge-\frac{9}{2}\)

Suy ra Min N = -9/2 <=> x = 1/2

Ta có :  (n-2)! = 1.2.3...(n-2) ; (n-1)! = 1.2.3...(n-2).(n-1) = (n-2)! . (n-1)

\(\frac{1}{\left(n-1\right)!}+\frac{1}{\left(n-2\right)!}=\frac{\left(n-2\right)!+\left(n-1\right)!}{\left(n-1\right)!.\left(n-2\right)!}=\frac{\left(n-2\right)!+\left(n-2\right)!.\left(n-1\right)}{\left(n-1\right)!.\left(n-2\right)!}\)

\(=\frac{\left(n-2\right)!\left(1+n-1\right)}{\left(n-1\right)!.\left(n-2\right)!}=\frac{n}{\left(n-1\right)!}\)

Đề bài của bạn sai.

Cách 1: A = { 9, 10, 11, 12, 13 }

Cách 2: A = { x \(\in\) N / 8 < x < 14 }

12 \(\in\) A  

16 \(\notin\) A

Cho mình **** nha.

1. How many / books / you / got ?

=> How many books did you get?

2. Mother / go / shopping / twice / week.

=> My mother goes shopping twice a week.

3. They / come / stay / us / next week.

=> They will come to stay with us next week.

4. The Vietnamese people / speak / English.

=> The Vietnamese people speak English

5. They / go / buy / new / house

=> They go to buy a new house.

1. The bus doesn't  arrive at 8.30 AM

2. The cat doesn't  like me.

1. Brother live in a small house in HCM city

2. Our school is large , but their school is small

3. There are three pictures on the wall in Tan's room

4. Mr. Hang is a doctor and he/she is thirty years old

5. Hoa has breakfast and goes to school at seven o'clock

1. How many classrooms are there in her school?

2. Hoa's house is big and it's in the country

3. Peter goes to school at two o'clock every afternoon

4. Does Hoa live in the city or in the the country?

5. ???

6. I'm in grade 10 and I'm in class 10D

1. What time do you go to school?

=> I go to school at 7 AM

2. Where do they live?

=> They live in the country.

3. Where does she live?

=> She lives in the city

4. How many teachers does your school have?

=> My school has 30 teachers

5. How many teachers are there in her school?

=> There are 45 teachers in her school.

6. What is there behind your school?

=> It is tall tree.

7. What does she do?

=> She is a doctor

8. What's your job?

=> I'm an engineer.

a/ \(x^2+2x+3=\left(x^2+2x+1\right)+2=\left(x+1\right)^2+2\ge2>0\) với mọi số thực x

b/ \(A=\left(x-1\right)\left(x-3\right)+11=x^2-4x+14=\left(x^2-4x+4\right)+10=\left(x-2\right)^2+10\ge10\)

Suy ra Min A = 10 <=> x = 2

\(B=\left(x^2-3x+1\right)\left(x^2-3x-1\right)\)

Đặt \(t=x^2+3x\)  thì \(B=t^2-1\ge-1\)

Do đó Min B = -1 <=> t = 0 <=> \(\left[\begin{array}{nghiempt}x=0\\x=-3\end{array}\right.\)

c/\(C=5-4x^2+4x=-\left(4x^2-4x+1\right)+6=-\left(2x-1\right)^2+6\le6\)

Suy ra Max C = 6 <=> x = 1/2

\(D=-x^2-4x-y^2+2y=-\left(x^2+4x+4\right)-\left(y^2-2y+1\right)+5\)

\(=-\left(x+2\right)^2-\left(y-1\right)^2+5\le5\)

Suy ra Max D = 5 <=> (x;y) = (-2;1)

Bài 4.

1. Why is not this TV working?

=> What happens to this TV?

2. Hoa nd Minh didn't go to school yesterday beause of illness

=> Hoa and Minh were ill , so they didn't go to school yesterday.

3. Can't you do better than that?

=> Is that the best you did?

4. It isn't necessary for students to wear uniforms on Wednesday.

=> Students need to wear uniforms on Wednesday.

5. She feels comfortable to watch TV

=> Watching TV makes her comfortable