1. There used to be a church near the post office in my town.

2. While I was writting the letter, the phone rang.

3. I'm very pleased to see Tom again after a long time.

4. You look thinner. Did you lose weight?

5. Do you think you can make the radio work again?

6. I bought the book 5 days ago.

7. A man said to the girl that there was a nearby cafe.

8. When I saw Nancy yesterday, she was driving a car.

9. They have known each other for 3 months.

10. Nam's teacher has taught him since he came here in 2000.

a^2-b^2-c^2-ab-ac-bc

=2a^2-2b^2-2c^2-2ab-2ac-2bc

=(a^2-2ab+b^2)+(b^2-2bc+c^2)+(a^2-2ac+c^2)

=(a-b)^2+(b-c)^2+(a-c)^2

Ta có (a-b)^2 lớn hơn 0 hoặc bằng 0.        (b-c)^2 lớn hơn hoặc bằng 0

           (a-c)^2 lớn hơn hoặc bằng 0

=>(a-b^2+(b-c)^2+(a-c)^2 lớn hơn hoặc bằng 0

vậy a^2+b^2+c^2-ab-ac-bc lớn hơn hoặc bằng 0

x chia hết cho 8,10,15

=>x thuộc BC(8;10;15)

BCNN (8;10;15)=120

=>x thuộc B(120)={0;120;240;..}

vì 100<x<200 nên x= 120

C. Nhỏ hơn 500N

a) 18.17 - 3.6.7

= 18.17 - 18.7

= 18.(17 - 7 )

= 18 . 10

= 180

b) 54 - 6.(17 + 9)

= 6.9 - 6.17 + 6.9

= (6.9 - 6.9) - 6.17

= 0 - 6.17

= 0 - 102

= -102

c) 33.(17 - 5) - 17.(33 - 5)

= 33.17 - 33.5 - 17 . 33 - 17.5

= 33.(17 - 17) - 5.(33 - 17 )

= 33. 0 - 5.16

= 0 - 80

= -80