15cm 

Nhân chia trước cộng trừ sau =3

3

1/ It is the custom in my country for children to receive special gifts on International Children's Day

2/ There is a tradition that parents hold a party and invite their relatives and close friends when their newborn baby turns 1 month old.

3/ Traditionally we give flowers to teachers on Vietnamese Teachers' Day

4/ Most of the people in my country follow the tradition of ancestor worship

5/ Some people break tradition by traveling instead of staying at home during Tet

6/ It's good for parents to have the habit of telling their children fairy tales before bedtime

7/ There is a custom of offering tea when guests come to visit their home in their country 

Chúc bạn học tốt :)

a/ HBr => H(I) và Br(I)

H₂S => H(I) và S(II)

CH₄ => H(I) và C(IV)

b/ Fe₂O₃ => Fe(III) và O(II)

CuO => Cu(II) và O(II)

Ag₂O => Ag(I) và O(II)

19.b

20a 

D. Nhé

B

C

D

Nhé

Các đơn chất là: S.Cl2.N2.Fe