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

\(\dfrac{8}{13}+\dfrac{4}{9}+\dfrac{1}{3}+\dfrac{5}{13}+3\)

\(=1+\dfrac{7}{9}+3\)

\(=4+\dfrac{7}{9}=\dfrac{43}{9}\)

DAY LA TOAN LOP 4 MA!

A)\(\frac{3}{5}\times\frac{7}{9}+\frac{11}{9}\times\frac{3}{5}-\frac{3}{10}\\ =\frac{3}{5}\times\left(\frac{7}{9}+\frac{11}{9}\right)-\frac{3}{10}\\ =\frac{3}{5}\times\frac{18}{9}-\frac{3}{10}=\frac{3\times2}{5}-\frac{3}{10}\)

   \(=\frac{6}{5}-\frac{3}{10}=\frac{12}{10}-\frac{3}{10}=\frac{9}{10}\)

A  =\(\frac{2}{5}\times\frac{4}{7}+\frac{3}{5}\times\frac{4}{7}\)  B = \(\frac{17}{13}\times\frac{12}{17}-\frac{4}{13}\times\frac{12}{17}\)   C = \(\frac{9}{14}:\frac{6}{7}+\frac{5}{14}:\frac{6}{7}\)  D =\(\frac{9}{12}:\frac{7}{9}\times\frac{12}{19}\)

=  \(\frac{4}{7}\times\left(\frac{2}{5}+\frac{3}{5}\right)\)              =\(\left(\frac{17}{13}-\frac{4}{13}\right)\times\frac{12}{17}\)                = \((\frac{9}{14}+\frac{5}{14}):\frac{6}{7}\)         = \(\frac{9}{12}\times\frac{9}{7}\times\frac{12}{19}\)

=\(\frac{4}{7}\times1\)                                     = \(1\times\frac{12}{17}\)                                       = \(1:\frac{6}{7}\)                               = \(\frac{9\times9\times12}{12\times7\times19}\)

=\(\frac{4}{7}\)                                              =  \(\frac{12}{17}\)                                                =\(\frac{7}{6}\)                                    = \(\frac{9\times9}{7\times19}=\frac{81}{133}\)

   -  

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 : \(1+2+3+...+19+20\)

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

Bài 4: 

a: \(=\dfrac{11}{21}\left(\dfrac{7}{4}+\dfrac{5}{4}\right)=\dfrac{11}{21}\cdot3=\dfrac{11}{7}\)

b: \(=\dfrac{6}{13}\left(\dfrac{23}{14}-\dfrac{9}{14}\right)=\dfrac{6}{13}\)

Bài 4: 

a: =11/21(7/4+5/4)=11/21x3=11/7

b: =6/13(23/14-9/14)=6/13

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

=(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+10

=20*9+10

=180+10=190

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

= 19 - 1( tức là trừ đi số 10 vì nó tròn chục )

=  18 : 2

=  9 x ( 19 + 1 ) + 10

=  9 x 20 + 10

=   180 + 10

=    190

tick cho bạn bách trả lời là 190 đi ai tick cho cậu ấy mình sẽ đội ơn người đấy