\(\dfrac{9}{14}+\dfrac{19}{18}+\dfrac{19}{14}+\dfrac{35}{18}=\left(\dfrac{9}{14}+\dfrac{19}{14}\right)+\left(\dfrac{19}{18}+\dfrac{35}{18}\right)=\dfrac{28}{14}+\dfrac{54}{18}=2+3=5\)

\(=\left(\dfrac{9}{14}+\dfrac{19}{14}\right)+\left(\dfrac{19}{18}+\dfrac{35}{18}\right)=2+3=5\)

\(\dfrac{9}{14}+\dfrac{19}{18}+\dfrac{19}{14}+\dfrac{35}{18}\\ =\left(\dfrac{9}{14}+\dfrac{19}{14}\right)+\left(\dfrac{19}{18}+\dfrac{35}{18}\right)\\ =\dfrac{28}{18}+\dfrac{54}{18}\\ =2+3=5\)

Số số hạng là 19-11+1=9(số)

Tổng là: 10x9=90

=(11+19)+(12+18)+(13+17)+(14+16)+15

=30+30+30+30+15

=135

90

10-11+12-13+14-15+16-17+18-19+20
= (10+20)+(-11-19)+(12+18)+(-13-17)+(14+16)-15
= 30-30+30-30+30-15
= 15

(20 + 10) - (11 + 19) + ( 12 + 18 ) - ( 13 + 17 ) + ( 14 + 16 ) - 15

= 30 - 30 + 30 - 30 - 15

= -15

(20+10)-(11+19)+(12+18)-(13+17)+(14+16)-15

=30-30+30-30-15+30

=15

a.      19 + 18 + 17 + 16 + 14 + 21 + 22 +  23 + 24 + 26

= (19 + 21) + (18 + 22) + (17 + 23) + (16 + 24) + (14 + 26)

= 40 + 40 + 40 + 40 + 40

= 40 x 5

= 200

\(11-12+13-14+15-16+17-18+19-20+21+22\)

\(=\left(11+13+15+17+19+21+22\right)-\left(12+14+16+18+20\right)\)

\(=\left(A+22\right)-B\)

     số số hạng của tổng A là :

         ( 21 - 11 ) :2 + 1 = 6 ( số )

    vậy tổng A là :

     ( 21 + 11 ) * 6 :2 = 96 

  số số hạng của tổng B là :

     (20-12 ) :2 +1 = 5 ( số )

tổng B là :

     ( 20 + 12 ) * 5 :2 = 80 

   vậy \(\left(A+22\right)-B\)

   \(=\left(96+22\right)-80\)

\(=118-80\)

\(=38\)

\(11-12+13-14+15-16+17-18+19-20+21+22\)

\(=\left(11-12\right)+\left(13-14\right)+\left(15-16\right)+\left(17-18\right)+\left(19-20\right)+21+22\)

\(=-1+-1+-1+-1+-1+21+22\)

\(=-5+21+22\)

\(=38.\)

21-20+19-18+17-16+15-14+13-12+11+22=1+1+1+1+1+33=38

minh chiu

a,

19nhan (82+18)=19nhan100

                         =1900

b, thì mình chịu

1+2+3+4+5+6+7+8+9+10+11+12+13+14+15+16+17+18+19+20

=(1+19)+(2+18)+(3+17)+(4+16)+(5+15)+(6+14)+(7+13)+(8+12)+(9+11)+20+10

=20+20+20+20+20+20+20+20+20+20+10

=20×10+10

=200+10

=210

= ( 1 + 19 ) + ( 2 + 18 ) + ( 3 + 17 ) + (4 + 16 ) + ( 5 + 15 ) + ( 6 + 14 ) + ( 7 + 13 ) + ( 8 + 12) + ( 9+ 11 ) + 20

= 20 + 20 + 20 +20 +  20 +20 + 20 + 20 + 20 +20

= 20 x 10

= 200

tui đầu tiên đó 

Ta có : \(1+2+3+...+19+20\)

          \(=\frac{\left(20+1\right).20}{2}=\frac{21.20}{2}=210\)

(19+1)+(18+2)+(17+3)+(16+4)+(15+5)+(14+6)+(13+7)+(12+8)+(11+9)+20)+10

=20+20+20+20+20+20+20+20+20+20=(20*10)+10

=210

1+2+3+4+5+6+7+8+9+10+11+12+13+14+15+16+17+18+19+20

=(1+19)+(2+18)+(3+17)+(4+16)+(5+15)+(6+14)+(7+13)+(8+12)+(9+11)+(10+20)

=20+20+20+20+20+20+20+20+20+30

=(20 x 9) + 30

=180 + 30

=210

tk nha

1+2+3+4+5+6+7+8+9+10+11+12+13+14+15+16+17+18+19+20

= (1+19)+(2+18)+(3+17)+(4+16)+(5+15)+(6+14)+(7+13)+(8+12)+(9+11)+20+10

= 20     +   20   +  20     +  20    +  20      +  20    +  20     +  20    +  20     +  20 + 10

= 10 . 10 +10 

=  100     +  10 = 110

(11+19)+(12+18)+(13+17)+(14+16)+(15+20)

=30+30+30+30+35

=155