a) Δ' = b'² - ac = [-(n-1)]² - 2n + 3

= n² - 2n + 1 - 2n + 3

= n² - 4n + 4 = ( n - 2 )² ≥ 0 ∀ n

hay pt luôn có nghiệm ∀ n (đpcm)

b) Theo Viète ta có : \(\hept{\begin{cases}x_1+x_2=-\frac{b}{a}=2n-2\\x_1x_2=\frac{c}{a}=2n-3\end{cases}}\)

Khi đó P = x₁² + x₂² = ( x₁ + x₂ )² - 2x₁x₂

= ( 2n - 2 )² - 2( 2n - 3 )

= 4n² - 8n + 4 - 4n + 6

= 4n² - 12n + 10

= ( 2n - 3 )² + 1 ≥ 1 ∀ n

Dấu "=" xảy ra <=> n = 3/2 . Vậy MinP = 1

Nếu chuyển từ kho thứ nhất sang kho thứ 2 15 tấn thóc thì số thóc ở kho 2 hơn kho 1: 6 tấn thó

 => lúc đầu kho 2 hơn kho 1:

  15+6=21( tấn thóc)

Lúc đầu kho 1 chứa:

  (216+21);2+15=`113,5(tấn)

Lúc đầu kho 2 chứa:

  216-113,5=102,5(tấn)

  Đ/s:.............

hmm mik nghĩ bạn sai 

Hello, my name is ... Today I will tell you about my favorite TV in the future. It's a wireless TV that can make calls, play games, place orders and communicate and watch alien broadcasts. It will also be very comfortable when it comes to helping us watch movies or look up homework. And alien sports matches will also be broadcast on this wireless TV.

(bài này có vẻ chán :P)

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DSXC

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\(\frac{11}{12}+\frac{7}{16}\div\frac{-3}{4}-\frac{13}{12}=\)\(\frac{11}{12}+\frac{-7}{12}-\frac{13}{12}=\)\(\frac{-3}{4}\)

\(\left(\frac{1}{2}-\frac{2}{3}\right)\times\frac{4}{5}\)

\(=\frac{-1}{6}\times\frac{4}{5}\)

\(=\frac{-2}{15}\)

\(\left(\frac{-4}{9}\right)\times\frac{3}{8}+\frac{1}{18}\)

\(=\frac{-1}{6}+\frac{1}{18}\)

\(=\frac{-1}{9}\)

\(\frac{2}{7}+\frac{28}{27}\div\left(\frac{-4}{9}\right)\)

\(=\frac{2}{7}+\frac{-7}{3}\)

\(=\frac{-43}{21}\)

1, D

2,D

3, A

4, A

5,D

6, A

7, C

8,  A

9, D

10, A

1. D

2.D 

3. A

4. A

5.D

6. A

7.C

8.A

9. D

10. A

nhỡ sai cũng cho luôn à anh 

Chu vi AMND=36 cm => cạnh AM=DN= 36/4=9cm

=> AB=9+5= 14cm

Tỉ số diện tích AMND/ABCD= AM.MN/AB.BC= 9.9/14.9=9/14

a) x=2

b) x=6

a)\(3.\left(5^x-1\right)-2=70\\3.\left(5^x-1\right)=72\\5^x-1=24\\ 5^x=25=5^2 \\ \Rightarrow x=2 \)

b)\(2^x+2^{x+1}+2^{x+2}=960-2^{x+3}\\2^x+2^{x+1}+2^{x+2}+2^{x+3}=960\\ 2^x\left(1+2+4+8\right)=2^x.15=960\\ 2^x=64=2^6\\ x=6\)

1.You / smoke / you / get yellow teeth

=> If you smoke, you get yellow teeth

2.Children / play outside / they / not get overweight. 

=> If the children not play outside, they not get overweight

3.You / heat ice / it / melt.

=> If you heat ice, it melt

4.I /speak to john / he / get annoyed. 

=> If I speak to John, he get annoyed

5.Lots of people / come/ Jenny / have a party.

=> if lots of people come, Jenny has a party 

6.I / feel good the next day / I / go to bed early.

=> I feel good the next day, if I go to bed early

7.She / buy expensive clothes / she / go shopping.

=> She buy expensive clothes if she go shopping

8.My daughter / pass her exams / she / work hard.

=> My daughter passed her exam because she had worked hard.

9.David / be sick / he / drink milk

=> David'll be sick if he drinks milk.

10.The river / freeze / it / be very cold.

=> The river froze because it was very cold.

11.I / to visit the museums / I / be in a new city.

=> I would to visit the museums if I was in a new city.

12.I / cycle to work / the weather / be fine.

=> I will cycle to work if the weather is fine.

13.My flatmate / clean it really well / she / clean the house.

=> My flatmate cleans it very well when she is cleaning the house.

14.Everybody / be grumpy / it / rain a lot.

=> Everybody will be grumpy if it rains a lot.

1. If you smoke, you'll get yellow tooth.

2. Children should play outside if they don't want to get overweight.

3. If you heat ice, it melts.

4. If I speak to John, he'll be annoyed.

5. Lots of people came to Jenny's house to have a party.

6. If I want to feel good the next day, I should go to bed early.

7. She bought expensive clothes while she was going shopping.

8. My daughter passed her exam because she had worked hard.

9. David'll be sick if he drinks milk.

10. The river froze because it was very cold.

11. I would to visit the museums if I was in a new city.

12. I will cycle to work if the weather is fine.

13. My flatmate cleans it very well when she is cleaning the house.

14. Everybody will be grumpy if it rains a lot.