A=(2ab-a^2-b^2+c^2).(2ab+a^2+b^2-c^2)

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

A=(c-a+b)(c+a-b)(a+b-c)(a+b+c)

Do c+b-a>0

c+a-b>0

a+b-c>0

a+b+c>0

=>A>0

@Hà Nhung Huyền Trang

Phân tích vế trái ta được: 2(x² + y² + z² − (xy + yz + zx)

Phân tích vế phải ta được: 6(x² + y² + z² − (xy + yz + zx)

Vì VT = VP nên VP - VT=0

→ 4(x² + y² + z² − (xy + yz + zx)) = 0

→2(2 (x² + y² + z² − (xy + yz + zx))) = 0

→2((x − y)² + (y − z)² + (z − x)²) = 0

→(x − y)² + (y − z)² + (z − x)² = 0

→(x − y)² = 0; (y − z)² = 0; (z − x)² = 0

→x = y = z

CM: a = -3b

Theo đề bài, ta có: a-b = 2(a+b) 

=> a-b = 2a + 2b 

=> a - 2a = 2b + b

=> a(1 - 2) = b(2 + 1)

=> a.(-1) = b.3

=> -a = 3b

=> a = -3b

đáp án D là đúng bạn nhé

Đáp án A là đúng bạn nhé

6. travel

7. doesn't do

(+) My mother has cooked dinner in the kitchen.

(-) I haven't eaten cookies for 2 hours.

(?) Has she seen him since 20 minutes.

-"What": What is the date today?

-"Where": Where did you go last summer?

-"How": How do you get there?

-"Who": Who ate them?

-"When": When does she go to the party?

2^x+1 - 2^{x = 6}

2^x.2¹ - 2^x.1 = 6

2^x.(2¹ - 1) = 6

2^x.1 = 6

2^x = 6:1

2^x = 6

2^x = 2.3

=> x \(\in\) \(\varnothing\)

2/7x5-11/14=9/14