Ta có:

3⁴ⁿ⁺¹ = ...3

3⁴ⁿ⁺² = ...3 . 3 = ...9

Mà 2014 chia 4 dư 2

=> 3²⁰¹⁴ = ...9

\(\left(\frac{2}{1.2.3}+\frac{2}{2.3.4}+...+\frac{2}{19.20.21}\right).x=5\)

\(\left(\frac{3-1}{1.2.3}+\frac{4-2}{2.3.4}+...+\frac{21-19}{19.20.21}\right).x=5\)

 \(\left(\frac{1}{1.2}-\frac{1}{2.3}+\frac{1}{2.3}-\frac{1}{3.4}+...+\frac{1}{19.20}-\frac{1}{20.21}\right).x=5\)

 \(\left(\frac{1}{1.2}-\frac{1}{20.21}\right).x=5\)

 \(\frac{209}{420}.x=5\)

\(\Rightarrow x=5\div\frac{209}{420}=\frac{2100}{209}\)

\(\left(\frac{2}{1.2.3}+\frac{2}{2.3.4}+...+\frac{2}{19.20.21}\right).x=5\)

\(\left(\frac{1}{1.2.3}+\frac{1}{2.3.4}+...+\frac{1}{19.20.21}\right).2.x=5\)

\(\left(\frac{1}{2}.\left(\frac{1}{1.2}-\frac{1}{2.3}\right)+\frac{1}{2}.\left(\frac{1}{2.3}-\frac{1}{3.4}\right)+...+\frac{1}{2}.\left(\frac{1}{19.20}-\frac{1}{20.21}\right)\right).x.2=5\)

\(\left(\frac{1}{2}.\left(\frac{1}{1.2}-\frac{1}{2.3}+\frac{1}{2.3}-\frac{1}{3.4}+...+\frac{1}{19.20}-\frac{1}{20.21}\right)\right).x=5\div2\)

\(\left(\frac{1}{2}.\left(\frac{1}{1.2}-\frac{1}{20.21}\right)\right).x=2,5\)

\(\left(\frac{1}{2}.\left(\frac{1}{2}-\frac{1}{420}\right)\right).x=2,5\)

\(\left(\frac{1}{2}\times\frac{209}{420}\right)\times x=2,5\)

\(\frac{209}{840}\times x=2,5\)

\(x=2,5\div\frac{209}{840}=10\frac{10}{209}\)

a, (23. x + 45) : 6 = 19                                           b , (x : 23 + 45) . 67 = 6911

    (23 . x + 45) = 19 x 6                                              ( x : 23 + 45) = 6911 : 67 

   (23 . x + 45) = 114                                                   ( x : 23 + 45) = 6911/67 

    23 . x = 114 - 45                                                      x : 23 = 6911/67 - 45

    23 . x = 69                                                              x : 23 = 3896/67 

     x = 69 : 23                                                               x = 3896/67 x 23 

    x =3                                                                         x = 89608/67

c,  x + (x + 1) + (x + 2) + ... + (x + 2011) = 2025078 

   x + x + x + ... +  x + (1 + 2 + ... + 2011) = 2025078 

  2012x + 2024072 = 2025078 

  2012x = 2025078 - 2024072 

  2012x = 1006 

=> x = 1/2 = 0,5

a) (23.x+45):6 = 19

23.x+45 = 19 . 6

23.x+45 = 114

23x = 114 - 45

23x = 69

x = 63 : 23

x = 3

b) (x:23+45) . 67 = 6911

x:23+45 = 6911 : 67

x:23+45 = \(\frac{6911}{67}\)

x:23 = \(\frac{6911}{67}-45\)

x:23 = \(\frac{3896}{67}\)

x = \(\frac{3896}{67}.23\)

x = \(\frac{89608}{67}\) 

c) x + ( x + 1 ) + ( x + 2 ) + .....+ (x + 2011) = 2025078

=> có 2011 cặp và 1 số x

=> (x+x+x+...+x) + ( 1 + 2 + ... + 2011 ) = 2025078

=> 2012x + 2023066 = 2025078

2012x = 2025078 - 2023066 

2012x = 2012

x = 2012 : 2012

x = 1

a)

3,2 . x + ( -1,2 ) . x + 2,7 = -4,9

x . ( 3,2 - 1,2 ) + 2,7 = -4,9

x . 2 + 2,7 = -4,9

x . 2 = -4,9 - 2,7

x . 2 = -7,6

x = -7,6 : 2

x = -3,8

b) ( -5,6 ) . x + 2,9 . x - 3,86 = -9,8

x . ( -5,6 + 2,9 ) - 3,86 = -9,8

x . ( -2,7 ) - 3,86 = -9,8

x . ( -2,7 ) = -9,8 + 3,86

x . ( -2,7 ) = -5,94

x = ( -5,94 ) : ( -2,7 )

x = 2,5

a) 3,2 x + (-1,2) x +2,7 = -4,9

   x(3,2 +-1.2 ) +2,7 =-4,9

  x . 2 +2,7 =-4,9

   x.2 = -7,6

    x= -3,8.

b) (-5,6) . x + 2,9 x - 3,86 = -9,8

x (-5,6 + 2,9) - 3,86 = -9,8

x. -2,7 = -5,94

x =2,2

a) x=5

b) x=0 hoặc 1

c) x=2

d) x=6 hoặc 5

e) x=0 hoặc 1

f) x=8

a, 2^x . 4 = 128

2^x = 128 : 4

2^x = 32

2^x = 2⁵

=> x = 5

b, x¹⁵ = x¹

=> x¹⁵ - x = 0

x . ( x¹⁴ - 1 ) = 0

=> x = 0 hoặc x¹⁴ - 1 = 0 

=> x = 0 hoặc x = 1

c, (2x + 1)³ = 125

( 2x + 1 )³ = 5³

=> 2x + 1 = 5

=> 2x = 5 - 1

=> 2x = 4

=> x = 4 : 2

=> x = 2

d, (x – 5)⁴ = (x - 5)⁶

=> ( x - 5 )⁶ - ( x - 5 )⁴ = 0

=> ( x - 5 )⁴ . [ ( x - 5 )² - 1 ] = 0

=> \(\orbr{\begin{cases}\left(x-5\right)^4=0\\\left(x-5\right)^2-1=0\end{cases}\Rightarrow\orbr{\begin{cases}x=0\\x=6\end{cases}}}\)

e, x¹⁰ = x

x¹⁰ - x = 0

x . ( x⁹ - 1 ) = 0

\(\Rightarrow\orbr{\begin{cases}x=0\\x^9-1=0\end{cases}\Rightarrow\orbr{\begin{cases}x=0\\x=1\end{cases}}}\)

f, (2x -15)⁵ = (2x -15)³

( 2x - 15 )⁵ - ( 2x - 15 )³ = 0

( 2x - 15 )³ . [ ( 2x - 15 )² - 1 ] = 0

\(\Rightarrow\orbr{\begin{cases}\left(2x-15\right)^3=0\\\left(2x-15\right)^2-1=0\end{cases}\Rightarrow\orbr{\begin{cases}2x-15=0\\2x-15=1\end{cases}\Rightarrow}\orbr{\begin{cases}x\text{ không tồn tại}\\x=8\end{cases}}}\)

(50-1):1+1=50 số

=(50-49)+(48-47)+...+(4-3)+(2-1). Ta có 25 cặp số

=1+1+1+....+1

=1.25

=25

vậy còn phần B bạn ơi giải lun cho mk đi 

B={11;12;13;14;15;16;17;18;19;20;21;22;23;24;25;26;27;28;29} có (29-11):1+1=19(số)

Vậy đáp án đúng là B.19

B.19 mình chắc chắn 100000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000%

hi hanari lucy 

ta kết bạn rồi đó

nhanh lên nha

117; 469 không phải số nguyên tố bạn ơi

các số nguyên tố:131,313,647