Rút gọn biểu thức:

\(\sqrt{\frac{a+\sqrt{a^2-b}}{2}}+\sqrt{\frac{a-\sqrt{a^2-b}}{2}}\)

Tính 3 góc của tam giác ABC biết \(\sqrt{3}cosA+2cosB+2\sqrt{3}cosC=4\)

a,b,c là các số thực dương và a+b+c=3. Chứng minh rằng :

\(\sqrt{a}+\sqrt{b}+\sqrt{c}\ge ab+bc+ac\)

Read the following passage and mark the letter A, B, C or D on your answer sheet to indicate the correct answer to each of the questions below.

     Because writing has become so important in our culture, we sometimes think of it as more real than speech. A little thought, however, will show why speech is primary and writing secondary to language. Human beings have been writing (as far as we can tell from surviving evidence) for at least 5000 years; but they have been talking for much longer, doubtless ever since there have been human beings.

     When writing did develop, it was derived from and represented speech, although imperfectly. Even today there are spoken languages that have no written form. Furthermore, we all learn to talk well before we learn to write; any human child who is not severely handicapped physically or mentally will learn to talk. a normal human being cannot be prevented from doing so. On the other hand, it takes a special effort to learn to write. In the past many intelligent and useful members of society did not acquire the skill, and even today many who speak languages with writing systems never learn to read or write, while some who learn the rudiments of those skills do so only imperfectly.

     To affirm the primacy of speech over writing is not, however, to disparage the latter. One advantage writing has over speech is that it is more permanent and makes possible the records that any civilization must have. Thus, if speaking makes us human, writing makes us civilized.

The pronoun "they" in the second paragraph refers to ______________.

A. son

B. university graduates

C. Oxford and Cambridge universities

D. educational opportunities

Biện pháp dưới đây góp phần vào việc bảo vệ nguồn tài nguyên hoang dã là:

A. Không cày xới đất để làm ruộng nương trên sườn đồi dốc để tránh sạt lở, xói mòn 

B. Đẩy mạnh việc thuần hoá động, thực vật, lai tạo các dạng động, thực vật mới có chất lượng và chống chịu tốt 

C. Xây dựng các khu bảo tồn thiên nhiên 

D. Các biện pháp trên đều đúng

Chọn câu đúng.

A.  19 . 20 19 + 20 = 1 19 + 1 20

B.  6 23 11 = 6 . 23 + 11 11

C.  a a 99 = 100 a 99 a ∈ ℕ *

D.  1 15 23 = 1 . 23 15

Cho z 1 , z 2  là nghiệm phương trình 6 − 3 i + i z = 2 z − 6 − 9 i  và thỏa mãn z 1 − z 2 = 8 5 . Tìm giá trị lớn nhất của z 1 + z 2 .

A.  56 5

B.  28 5

C. 6

D. 5