a, 18.(-24) +  35.42 - 18.76  + 35.(-142)

=  [ 18.(-24) - 18.76] + (35.42 + 35(-142)]

= -18.[ 24 + 76]  - 35.( 142 - 42)

= -18.100  - 35. 100

= - 100.(18 + 35)

= - 100. 53

= - 5300

h,   104 : (-13) - [56 - 220 : (-4)]

  = 104 : (-13)  - [ 56 + 55]

= -8 - 111

= - 119 

c) 19 - 42.(-19) - 38.5

= 19 + 42.19 - 19.2.5

= 19 + 42.19 - 19.10

= 19.(1 + 42 - 10)

= 19.33

= 627

X²+X-3X-3=15

X²-2X-3=15

X²-2X=18

X.X-2X=18

X(X-2)=18

TH1

X=18

TH2

X-2=18

X=20

Vậy Xϵ{18;20}

   (\(x\) - 3).(\(x\) + 1) = 15

   \(x^2\) - 3\(x\) + \(x\) - 3 = 15

  \(x^2\) - 2\(x\) - 3 + 4 =  15 + 4

   \(x^2\) - 2\(x\)  + 1    = 19

  (\(x\) - 1)²            = 19

 \(\left[{}\begin{matrix}x-1=-\sqrt{19}\\x-1=\sqrt{19}\end{matrix}\right.\)

\(\left[{}\begin{matrix}x=1-\sqrt{19}\\x=1+\sqrt{19}\end{matrix}\right.\)

56 + 29

-10 < x ≤ 13

⇒ x ∈ {-9; -8; -7; ...; 7; 8; 9; 10; 11; 12; 13}

Tổng của chúng là:

-9 + (-8) + (-7) + ... + 7 + 8 + 9 + 10 + 11 + 12 + 13

= 10 + 11 + 12 + 13

= 46

Chọn D

Đáp án D.46

12n + 1 = 12n + 18 - 17

= 6(2n + 3) - 17

Để (12n + 1) ⋮ (2n + 3) thì 17 ⋮ (2n + 3)

⇒ 2n + 3 ∈ Ư(17) = {-17; -1; 1; 17}

⇒ 2n ∈ {-20; -4; -2; 14}

⇒ n ∈ {-10; -2; -1; 7}

Mà n là số tự nhiên

n = 7

\(\dfrac{9}{14}+\dfrac{8}{21}=\dfrac{27}{42}+\dfrac{16}{42}=\dfrac{43}{42}\)

\(\dfrac{13}{15}-\dfrac{7}{12}=\dfrac{52}{60}-\dfrac{35}{60}=\dfrac{17}{60}\)

\(\dfrac{9}{14}+\dfrac{8}{21}=\dfrac{27}{42}+\dfrac{16}{42}=\dfrac{43}{42}\)

\(\dfrac{13}{15}-\dfrac{7}{12}=\dfrac{52}{60}-\dfrac{35}{60}=\dfrac{17}{60}\)

#hn

Ta có: \(2^2+2^3+2^4+2^5\)

\(=\left(2^2+2^3\right)+\left(2^4+2^5\right)\)

\(=12+2^2.\left(2^2+2^3\right)\)

\(=12+2^2.12\)

\(=12.\left(1+2^2\right)\)

Vì \(12⋮3\) nên \(12.\left(1+2^2\right)⋮3\)

Vậy \(2^2+2^3+2^4+2^5⋮3\)

CM: Tổng \(2^2+2^3+2^4+2^5⋮3\)

= \(2^2.\left(1+2\right)\) \(+2^4.\left(1+2\right)\)

= \(2^2.3+2^4.3\)

= \(3.\left(2^2+2^4\right)\)\(⋮3\)

Ta có: 

\(x-6⋮3\)

\(\Rightarrow x⋮3\) và \(x\in B\left(3\right)\)

Xét:

\(12⋮3\) ; \(19⋮̸3\) ; \(45⋮3\) ; \(70⋮̸3\)

⇒ \(x\in\left\{12;45\right\}\)