Ta có:

b=3a => b chia hết cho 3 => tổng các chữ số của b chia hết cho 3 mà tổng các chữ số của b= tổng các chữ số của a => a chia hết cho 3. Ta có 3 chia hết cho 3, a chia hết cho 3 nên 3a chia hết cho 9 => b chia hết cho 9 => tổng các chữ số của b chia hết cho 9 => a chia hết cho 9 vì tổng các chữ số của a = tổng các chữ số của b( đpcm)

Cho hai dao động điều hòa cùng phương có các phương trình lần lượt là:  x 1 = 4 cos πt - π 6 c m và  x 2 = 4 cos πt - π 2 c m . Dao động tổng hợp của hai dao động này có biên độ là

A. 8cm

B. 2cm

C .   4 3 c m

D .   4 2 c m

baby

Mark the letter A, B, C or D on your answer sheet to indicate the word whose underlined part differs from the other three in pronunciation in each of the following questions.

A. founded 

B. laughed 

C. crooked 

D. attracted

Read the following passage and mark the letter A, B, C or D on your answer sheet to indicate the correct word or phrase that best fits each of the numbered blanks from 17 to 21.

          Nowadays people are more aware that wildlife all over the world is in danger. Many species of animals are threatened, and could easily become (17) ______if we do not make an effort to protect them. There are many reasons for this. In some cases, animals are hunted for their fur or for other valuable parts of their bodies.

          Some birds, (18) ______as parrots are caught alive and sold as pets. For many animals and birds, the problem is that their habitat - the place where they live - is disappearing. More (19) ______is used for farms, for houses or industry, and there are fewer open spaces than there once were. Farmers use powerful chemicals to help them (20) ______ better crops, but these chemicals pollute the environment and harm wildlife. The most successful animal on earth - human beings - will soon be the only ones left, (21) ______ we can solve this problem.

Điền ô số 17

A. extinctive         

B. extinction          

C. extinct     

D. extinctions

Trong quá trình chuyển động của vật được ném ngang: 

A. Động năng không thay đổi.

B. Thế năng không đổi.

C. Cơ năng bảo toàn.

D. Động lượng bảo toàn.

 Although speech is the most advanced form of communication, there are many ways of communicating without using speech. Signals, signs, symbols, and gestures may be found in every known culture. The basic function of signal is to impinge upon the environment in such a way that it attracts attention, for example, the dots and dashes of a telegraph circuit. Coded to refer to speech, the potential for communication is really great. Less adaptable to the codification of words, signs also contain meaning in and of themselves. A stop sign or barber pole conveys meaning quickly and conveniently. Symbols are more difficult to describe than either signals or signs because of their intricate relationship with the receiver’s cultural perceptions. In some culture, applauding in a theater provides performers with an auditory symbol of approval. Gestures such as waving and handshaking also convey certain cultural messages.

          Although signals, signs, symbols, and gestures are very useful, they do have a major disadvantage. They usually do not allow ideas to be shared without the sender being directly adjacent to the receiver. As a result, means of communication intended to be used for long distances and extended periods are based upon speech. Radio, television, and the telephone are only a few.

The word “ it” in paragraph 1 refers to______.

A. way        

B. environment      

C. speech     

D. signal

D

100 phân số nha bạn

C