-19/7 < 0

1/6 > 0

Quãng đường ô tô đi từ Thái Bình đến Quảng Ninh:

363 - 206 = 157 (km)

a) -2/5 + 7/21

= -2/5 + 1/3

= -6/15 + 5/15

= -1/15

b) 3/8 + (-5/6)

= 9/24 + (-20/24)

= -11/24

3)

a) 7x + 49 = 28

7x = 28 - 49

7x = -21

x = -21 : 7

x = -3

Vậy x = -3

b) -3/2 + 5/2 x = 7/2

5/2 x = 7/2 + 3/2

5/2 x = 5

x = 5 : 5/2

x = 2

Vậy x = 2

c) 1/3 - 1/6 x = 1

1/6 x = 1/3 - 1

1/6 x = -2/3

x = -2/3 : 1/6

x = -4

Vậy x = -4

d) -7/4 - 3/2 x = 3/8

3/2 x = -7/4 - 3/8

3/2 x = -17/8

x = -17/8 : 3/4

x = -17/6

Vậy x = -17/6

e) (x - 1)/4 = (2x + 1)/10

5(x - 1) = 2(2x + 1)

5x - 5 = 4x + 2

5x - 4x = 2 + 5

x = 7

Vậy x = 7

Hiệu số phần bằng nhau: 3 - 2 = 1

Chiều dài: 15 × 3 : 1 = 45 (m)

Chiều rộng: 15 × 2 : 1 = 30 (m)

Chu vi: (45 + 30) × 2 = 150 (m)

Số đó là: 1023457

a) ĐKXĐ:

x - 2 > 0

\(\Leftrightarrow x>2\)

b) \(\sqrt{4x^2-4x+1}=\sqrt{\left(2x-1\right)^2}\)

Do \(\left(2x-1\right)^2\ge0\) với mọi \(x\in R\)

Vậy ĐKXĐ: với mọi \(x\in R\)

c) \(\sqrt{x^2-x-6}=\sqrt{\left(x-3\right)\left(x+2\right)}\)

ĐKXĐ:

\(\left[{}\begin{matrix}\left\{{}\begin{matrix}x-3\ge0\\x+2\ge0\end{matrix}\right.\\\left\{{}\begin{matrix}x-3< 0\\x+2< 0\end{matrix}\right.\end{matrix}\right.\Leftrightarrow\left[{}\begin{matrix}\left\{{}\begin{matrix}x\ge3\\x\ge-2\end{matrix}\right.\\\left\{{}\begin{matrix}x< 3\\x< -2\end{matrix}\right.\end{matrix}\right.\Leftrightarrow\left[{}\begin{matrix}x\ge3\\x< -2\end{matrix}\right.\)

d) ĐKXĐ:

x - 1 < 0

\(\Leftrightarrow x< 1\)

Ta có: A = {-1; 3}

B = {-2; -1; 2}

B \ A = {-2; 2}

13) 25a² - 49b⁴ = (5a)² - (7b²)² = (5a - 7b²)(5a + 7b²)

14) 100a² - 9b⁴ = (10a)² - (3b²)² = (10a - 3b²)(10a + 3b²)

15) a⁴ - 4b² = (a²)² - (2b)² = (a² - 2b)(a² + 2b)

16) 1/4 a² - b² = (1/2 a)² - b² = (1/2 a - b)(1/2 a + b)

17) 1/4 a² - 1/9 b² = (1/2 a)² - (1/3 b)² = (1/2 a - 1/3 b)(1/2 a + 1/3 b)

18) 4/9 a⁴ - 25/4 = (2/3 a²)² - (5/2)²

= (2/3 a² - 5/2)(2/3 a² + 5/2)

19) 25a² - 1/4 b² = (5a)² - (1/2 b)²

= (5a - 1/2 b)(5a + 1/2 b)

20) 1/25 - 36x² = (1/5)² - (6x)² = (1/5 - 6x)(1/5 + 6x)

21) (a - b)² - c² = (a - b - c)(a - b + c)

22) (a + b)² - 4 = (a + b)² - 2²

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

23) (a - 2b)² - 4b² = (a - 2b)² - (2b)²

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

= a(a - 4b)

24) (a + 3b)² - 9b² = (a + 3b)² - (3b)²

= (a + 3b - 3b)(a + 3b + 3b)

= a(a + 6b)

25) (a - 5b)² - 16b² = (a - 5b)² - (4b)²

= (a - 5b - 4b)(a - 5b + 4b)

= (a - 9b)(a - b)

26) 25a² - (a - b)² = (5a)² - (a - b)²

= (5a - a + b)(5a + a - b)

= (4a + b)(6a - b)

27) 4a² - (a - b)² = (2a)² - (a - b)²

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

= (a + b)(3a - b)

28) 49a² - (2a - b)² = (7a)² - (2a - b)²

= (7a - 2a + b)(7a + 2a - b)

= (5a + b)(9a - b)

29) 36a² - (3a - 2b)² = (6a)² - (3a - 2b)²

= (6a - 3a + 2b)(6a + 3a - 2b)

= (3a + 2b)(9a - 2b)

30) 81a² - (5a - 3b)² = (9a)² - (5a - 3b)²

= (9a - 5a + 3b)(9a + 5a - 3b)

= (4a + 3b)(14a - 3b)