=312x(17+13)

=312x20

=6240

=312 x ( 17 + 13 )

=312 x 20 

= 6240

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

Please

a. 312 x 425 + 312 x 574 + 312 = 312 × (425 + 574 + 1)

= 312 × 1000

= 312000

dễ ẹt

bằng 312000 chứ gì

dễ mà dơn giản

a. 312 x 425 + 312 x 574 + 312 = 312 × (425 + 574 + 1)

= 312 × 1000

= 312000

\(\dfrac{3}{7}\cdot\dfrac{9}{26}-\dfrac{1}{14}\cdot\dfrac{1}{13}\)

\(=\dfrac{3\cdot9-1}{14\cdot13}=\dfrac{26}{14\cdot13}=\dfrac{2}{14}=\dfrac{1}{7}\)

`4/6+7/13+17/9+19/13+1/9+14/6`

`=(4/6+14/6)+(7/13+19/13)+(17/9+1/9)`

`=18/6+26/13+18/9`

`=3+2+2`

`=7`

\(\frac{1}{2}\cdot\frac{18}{14}+\frac{1}{2}\cdot\frac{3}{14}-\frac{1}{2}\cdot\frac{7}{14}\)

\(=\frac{1}{2}\left(\frac{18}{14}+\frac{3}{14}-\frac{7}{14}\right)\)

\(=\frac{1}{2}\cdot1=\frac{1}{2}\)