\(a.145+86+14+55\)

\(=\left(145+55\right)+\left(86+14\right)\)

\(=200+100\)

\(=300\)

\(b.2356+1997+703\)

\(=2356+\left(1997+703\right)\)

\(=2356+2700\)

\(=5056\)

\(c.93273+398+727+5602\)

\(=\left(93273+727\right)+\left(398+5602\right)\)

\(=94000+6000\)

\(=100000\)

309 x 727 - 209 x 727

= (309 - 209) x 727

=100 x 727= 72 700

=(309-209)*727

=100*727

=72700

tính bằng cách thuận tiện nhất                                                                                        309 x 727 - 209 x 727

= (309 - 209) x 727

= 100 x 727

= 72700

309*727-209*727 nha

= 727*(309 - 209)

= 727*100

= 72700

64.6+81.4+17.6

=6.(64+17)+81.4

=6.81+81.4

=81.(6+4)

=81.10

=810

g) Ta có: \(64\cdot6+81\cdot4+17\cdot6\)

\(=6\left(64+17\right)+81\cdot4\)

\(=81\cdot6+81\cdot4\)

=810

64.6+81.4+17.6

=6.(64+17)+81.4

=6.81+81.4

=81.(6+4)

=81.10

=810

12 . 19 + 12

= 12 . 19 + 12 . 1

= 12 . ( 19 + 1 )

= 12 . 20

= 240

\(12\cdot19+12=12\left(19+1\right)=12\cdot20=240\)

=3,26(728+1+270)

=3,26*999=3256,74

\(3,26\cdot728+3,26+3,26\cdot270\)

\(=3,26\cdot\left(728+1+270\right)\)

\(=3,26\cdot999\)

\(=3,26\cdot\left(1000-1\right)\)

\(=3,26\cdot1000-1\cdot3,26\)

\(=3260-3,26\)

\(=3256,74\)

\(365365\div\left(91\times55\right)\)

=365365\(\div\) 5005

=73

`125+231+319+769+875+681`

`= (125+875) + ( 231+769) + ( 319+681 ) `

`= 1000 + 1000 + 1000 `

`= 3000`