Bài 3: 

a: \(=-8\left(72+19-1\right)=-8\cdot90-720\)

b: \(=-27\left(1011-12-1\right)=-27\cdot998=-26946\)

c: \(=17\cdot\left[29+111\right]+29\cdot\left(-17\right)\)

\(=17\left(29+111-29\right)=17\cdot111=1887\)

d: \(=43\cdot\left(-1\right)+40=-43+40=-3\)

a; 14 - 11 + 16 - 13 + 19 - 15 + 21 - 17

= (14 + 16) - (13 + 17) + (19 + 21) - (11 + 15)

= 30 - 30 + 40 - 26

= 0 + 40 - 26

= 14

b; 4 - 6  + 8 - 10 + 12  + 16 - 18 + 20

=  (8 + 12) - (6 + 10   - 16)   +(4 + 20 - 18)

= 20 - (16 - 16) + (24  - 18)

= 20 - 0 + 6

= 20 + 6

= 26 

a: \(A=21\cdot100-11\cdot100+90\cdot100+100\cdot125\cdot16\)

\(=100\left(21-11+90\right)+100\cdot2000\)

\(=100\left(10+90+2000\right)=2100\cdot100=210000\)

b: \(=\dfrac{5\cdot2^{30}\cdot3^{18}-2^{29}\cdot3^{20}}{5\cdot2^{28}\cdot3^{19}-7\cdot2^{29}\cdot3^{18}}\)

\(=\dfrac{2^{29}\cdot3^{18}\left(5\cdot2-3^2\right)}{2^{28}\cdot3^{18}\left(5\cdot3-7\cdot2\right)}=2\)

a) 145 + 43 + (-145) + 57

= [145 + (-145)] + [43 + 57]

= 0 + 100

= 100

A) = (145+(-145))+(43+57)

= 0 + 100

= 100

Bài 2: Tính một cách hợp lý:

a) 145 + 43 + (– 145) + 57;

= 145 - 145 + 43 + 57 = 0 + 100 = 100

b) (– 213) + 36 + (– 787) + 64;

= -213 - 787 + 36 + 64 = -1000 + 100 = -900

c) (– 2) + (– 4) + (– 6) + (– 10) + 8 + 10 + 12;

= -2 +12 - 4 - 6 - 10 + 10 + 8 = 10 - 10 -10 + 10 + 8 = 8 

d) (– 2 020) + (– 598) + (– 201) + 498 + 301.

= -2020 - 799 + 799 = -2020

=(1/13+12/13)+(2/13+11/13)+(3/13+10/13)+......(6/13+7/13)

=1+1+1+1+1+1

=6

\(=\frac{1+2+3+...+12}{13}\)

\(=\frac{\left(12+1\right)+\left(11+2\right)+..+\left(5+7\right)+6}{13}\)

\(=\frac{12+12+12+12+12+6}{13}\)

\(=\frac{12.5+6}{13}\)

\(=\frac{66}{13}\)

Cảm ơn bạn nhiều nhé!!!