Ta có: (a - b)² + (b - c)²  + (a - c)² = a² + b² + c²

<=> a² - 2ab + b² + b² - 2bc + c² + a² - 2ac + c² = a² + b² + c²

<=> a² + b² + c² = 2(ab + bc + ac)

<=> ab + bc + ac = \(\frac{a^2+b^2+c^2}{2}\) (1)

Ta lại có: a + b + c = 6

<=> (a + b + c)² = 36

<=> a² + b² + c² + 2(ab + bc + ac) = 36 

<=> a² + b² + c² + a² + b² + c² = 36 (vì a² + b² + c² = 2(ab + bc + ac)

<=> 2(a² + b² + c²) = 36 <=> a² + b² + c² = 18

<=> \(\frac{a^2+b^2+c^2}{2}=9\)(2)

Từ (1) và (2) => ab + ac + bc = 9

1.Complete with be going to.

1.Kelly...is going to have...a party for her birthday.(have)

2.Mike and Sarah............aren't comming.........................with us.(not/come)

3.When........................are you tidying........................your room?(you/tidy)

4.Mr and Mrs Allan............are moving.........................to Canada.(move)

5............Is your brother selling............................his car?(your borther/sell)

6.I..........amnot lending...............Ben my camera.(not lend)

7.We...........are staying...............at home on Sunday.(stay)

8.I..................amnot telling..............Anna about the party.(not/tell)

1.Kelly...is going to have...a party for her birthday.(have)

2.Mike and Sarah......are not going to come...............................with us.(not/come)

3.When....are .you..going to tidy.........................................your room?(you/tidy)

4.Mr and Mrs Allan..........are going to move...........................to Canada.(move)

5......When is your brother going to sell..................................his car?(your borther/sell)

6.I......am not going to lend..................Ben my camera.(not lend)

7.We...are going to stay.......................at home on Sunday.(stay)

8.I.....am not going to tell...........................Anna about the party.(not/tell)

D taị x=1/3

C = ( -5x + 4 )( 3x - 2 ) + ( -2x + 3 )( x - 2 )

C = -5x( 3x - 2 ) + 4( 3x - 2 ) - 2x( x - 2 ) + 3( x - 2 )

C = -15x² + 10x + 12x - 8 - 2x² + 4x + 3x - 6

C = -17x² + 29x - 14

Với x = -2 ta có :

C = -17.(-2)² + 29.(-2) - 14 = -140

D = ( x - 5 )( -3x + 1 ) - 3( x - 2 )( 2x - 1 )

D = x( -3x + 1 ) - 5( -3x + 1 ) - 3( 2x² - 5x + 2 )

D = -3x² + x + 15x - 5 - 6x² + 15x - 6

D = -9x² + 31x - 11

Với x = 1/3 ta có :

D = -9.(1/3)² + 31.1/3 - 11 = -5/3

Trả lời:

\(\sqrt{x^2-25}=\sqrt{\left(x-5\right).\left(x+5\right)}\)

                        \(=\sqrt{x-5}.\sqrt{x+5}\)

Học tốt

Bài làm:

Nửa chu vi HCN đó là:

20 : 2 = 10 (cm)

Tổng số phần bằng nhau giữa CD và CR là:

3 + 2 = 5 (phần)

Chiều dài HCN đó là:

10 : 5 x 3 = 6 (cm)

Chiều rộng HCN đó là:

10 - 6 = 4 (cm)

Diện tích HCN đó là:

6 x 4 = 24 (cm²)

Bg

Gọi chiều dài là a, chiều rộng là b  (a, b thuộc N*)

Nửa chu vi (a + b) của hình chữ nhật đó là:

   20 ÷ 2 = 10

Theo đề bài: a + b = 10 và a ÷ b = 3 ÷ 2

Vì a ÷ b = 3 ÷ 2 = 1,5 => a = b.1,5

=> b.1,5 + b.1 = 10

     b.(1,5 + 1)  = 10

     b.2,5           = 10

     b                 = 10 ÷ 2,5

=> b                 = 4 (cm)

=> a = 4.1,5 = 6 (cm)

Diện tích của hình chữ nhật đó là:

   6.4 = 24 (cm²)

1. Unless we try to save them, our sources of energy will soon end.

2. Mike doesn't play chess as well as Barbare.

3. Although the story's silly, many people enjoyed the film.

4. The ket price was high, however; many people went to see that film.

5. Despite the fact that he's young, he performs very well.

6. In spite of being a comedy, i didn't find it funny at all. 

7. The film was so boring, but Jack saw it from beginning to end.

8. Although the film has a happy ending, it begins with a terrible disaster.

9. Despite the fact that the film was a bit frightening, i really enjoyed it.

10. In spite of being young, he performs very well.

11. No river in the world is longer than the Nile.

12. We might visit Japan in the future.

13.My brother is a fast runner.

14.Nga's favourite sport is badminton.

\(\frac{10}{11}:\left(\frac{2}{3.5}+\frac{2}{5.7}+\frac{2}{7.9}+\frac{2}{9.11}\right)\)

\(=\frac{10}{11}:\left(\frac{1}{3}-\frac{1}{5}+\frac{1}{5}-\frac{1}{7}+\frac{1}{7}-\frac{1}{9}+\frac{1}{9}-\frac{1}{11}\right)\)

\(=\frac{10}{11}:\left(\frac{1}{3}-\frac{1}{11}\right)\)

\(=\frac{10}{11}:\frac{8}{33}=\frac{10}{11}.\frac{33}{8}\)

\(=\frac{15}{4}\)

Trả lời:

\(\frac{10}{11}\div\left(\frac{2}{3.5}+\frac{2}{5.7}+\frac{2}{7.9}+\frac{2}{9.11}\right)\)

\(=\frac{10}{11}\div\left(\frac{1}{3}-\frac{1}{5}+\frac{1}{5}-\frac{1}{7}+\frac{1}{7}-\frac{1}{9}+\frac{1}{9}-\frac{1}{11}\right)\)

\(=\frac{10}{11}\div\left(\frac{1}{3}-\frac{1}{11}\right)\)

\(=\frac{10}{11}\div\frac{8}{33}\)

\(=\frac{10}{11}\times\frac{33}{8}\)

\(=\frac{15}{4}\)