=>6x=30

hay x=5

7x - x = 30
7.x - 1.x = 30
x . (7-1) = 30
x . 6 = 30

x      = 30 : 6
x      = 5

DỄ ƠI LÀ DỄ

=\(3^3\left(22-19\right)\)

=\(3^3.3\)

=\(3^4\)=81

3^3.22-3^3.19 

= 3³ ( 22-19 )

= 3³ . 3

= 81

\(\left\{210:\left[16+3.\left(6+3.22\right)\right]\right\}-3\)

\(=\left\{210:\left[16+3.\left(6+66\right)\right]\right\}-3\)

\(=\left\{210:\left[16+3.72\right]\right\}-3\)

\(=\left\{210:\left[16+216\right]\right\}-3\)

\(=\left\{210:232\right\}-3\)

\(=\dfrac{105}{116}-3\)

\(=\dfrac{-243}{116}\)

\(7x-x=5^{21}:5^{19}+3.2^2-7^0\)

\(6x=5^2+3.2-1\)

\(6x=25+12-1\)

\(6x=36\)

\(x=6\)

K bt đk:))
Ta có :
7x - x = 5²¹ : 5¹⁹ . 3 . 2² - 70

=> 6x = 5²¹: 5¹⁹ + 3 . 2² - 70

=> 6x = 5² + 3. 4 - 1

=> 6x = 25 + 3 . 2 - 1

=> 6x = 25 + 6 - 1

=> 6x = 30

=> x = 30 : 6

=> x = 5

Đáp án A

y = x + x 2 + x 3 + ... + x 2018 = x 1 − x 2018 1 − x y ' = 1 + 2 x + 3 x 2 + ... + 2018 x 2017         = 2018 x 2019 − 2019 x 2018 + 1 ( 1 − x ) 2 y ' ( 2 ) = 1 + 2.2 + 3.2 2 + ... + 2018.2 2017 = 2017.2 2018 + 1

Đáp án là A