\(\frac{3}{5}\times\frac{6}{7}+\frac{3}{5}:7+\frac{6}{5}\)

\(=\frac{3}{5}\times\frac{6}{7}+\frac{3}{5}\times\frac{1}{7}+\frac{6}{5}\)

\(=\frac{3}{5}\times\left(\frac{6}{7}+\frac{1}{7}\right)+\frac{6}{5}\)

\(=\frac{3}{5}\times1+\frac{6}{5}\)

\(=\frac{3}{5}+\frac{6}{5}=\frac{9}{5}\)

~ Hok tốt ~

\(\frac{29}{12}:\frac{1}{2}-\frac{5}{12}:\frac{1}{2}-\frac{1}{2}\)

\(=\left(\frac{29}{12}-\frac{5}{12}\right):\frac{1}{2}-\frac{1}{2}\)

\(=2:\frac{1}{2}-\frac{1}{2}\)

\(=4-\frac{1}{2}=\frac{7}{2}\)

~ Hok tốt ~

\(\frac{2}{15}\times\frac{4}{7}+\frac{4}{7}\times\frac{2}{3}+\frac{4}{7}:5\)

\(=\frac{2}{15}\times\frac{4}{7}+\frac{4}{7}\times\frac{2}{3}+\frac{4}{7}\times\frac{1}{5}\)

\(=\left(\frac{2}{15}+\frac{2}{3}+\frac{1}{5}\right)\times\frac{4}{7}\)

\(=1\times\frac{4}{7}=\frac{4}{7}\)

~ Hok tốt ~

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

(7/15 + 8/15) + (9/16 + 7/16)

= 15/15 + 16/16

= 1 + 1

= 2

a) 145 + 86 + 14 + 55 = ( 145 + 55 ) + (86 + 14 )

= 200 + 100

= 300

b) 1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9 = ( 1+9 +2 + 8) + ( 3 + 7 + 4 + 6 ) + 5

= 20 + 20 + 5

= 45

39200

Ghi đầy đủ bài giải 

a,3/5*7/9+11/9*3/5-3/10=3/5*(7/9+11/9)-3/10

=3/5*18/9-3/10

=3/5*2-3/10

=6/5-3/10

=12/10-3/10

=9/10

b,14/13:1/2+3/13*2-2*7/13=14/13*2+3/13*2-2*7/13

=2*(14/13+3/13-7/13)

=2*10/13

=20/13

a/ ( 7/9 + 11/9 ) x 3/5 - 3/10

 =    2 x 3/5 - 3/10

=    6/5  - 3/10

 =    9/10

b/ 14/13 :  1/2 + 3/13 x 2 - 2 x 7/13

14/13 x 2/1 + 3/13 x 2 - 2 x 7/13

2 x ( 14/13 + 3/13 - 7/13 )

2 x 10/13

20/13

1*3+5*7+9*11+...+99*101

(2/7+5/7)+4/9=1+4/9=13/9

17/11+(7/15+5/11)=17/11+7/15+5/11=(17/11+5/11)+7/15=2+5/11=27/11

A = ( 2/7 + 5/7 ) + 4/9

A =      1 + 4/9

A = 13/9

B = ( 17/11 + 5/11 ) + 7/15

B =        2 + 7/15

B = 37/15

Chúc bạn học tốt~^^

A = 2/7 + 4/9 + 5/7 

A = ( 2/7 + 5/7 ) + 4/9 

A = 1 + 4/9 

A  = 13/9                             

Phần B làm tương tự bằng cách bỏ ngoặc nhé !

A = \(\left(\frac{2}{7}+\frac{4}{9}\right)+\frac{5}{7}\)

A = \(\frac{2}{7}+\frac{5}{7}+\frac{4}{9}\)

A = 1 + \(\frac{4}{9}\)

A = \(\frac{13}{9}\)

B = \(\frac{17}{11}+\left(\frac{5}{11}+\frac{7}{15}\right)\)

B = \(\frac{17}{11}+\frac{5}{11}+\frac{7}{15}\)

B = 2 + \(\frac{7}{15}\)

B = \(\frac{37}{15}\)

Nếu mình đúng các bạn k mình nhé