a) 8 × 5 × 9 = (8 × 5) × 9 = 40 × 9 = 360

b) 6 × 7 × 5 = (6 × 5) × 7= 30 × 7= 210

c) 6 × 4 × 25 = 6 × (4 × 25) = 6 × 100= 600

1 - 2 + 3 - 4 + 5 - 6 + ....... + 19 - 20

= ( -1 ) + ( -1 ) + (-1 ) + .......... + ( - 1)

= 10 . ( -1 )

= -10

1-2+3-4+5-6+.....+19-20 ( có 20 só hạng )

= - 1 + -1+-1+................+-1 ( có 10 số hạng )

= - 1 . 10 = -10

mình chép nhầm phía trên là đúng

\(\frac{1.3.5+2.6.10+4.12.20+7.21.35}{1.5.7+2.10.14+4.20.28+7.35.49}\)

\(=\frac{1.3.5\left(1+2+4+7\right)}{1.5.7\left(1+2+7+7\right)}=\frac{1.3.5}{1.5.7}=\frac{15}{35}=\frac{3}{7}\)

\(\frac{1\cdot3\cdot5+2\cdot6\cdot10+4\cdot12\cdot20+7\cdot21\cdot35}{1\cdot5\cdot7+2\cdot10\cdot14+4\cdot20\cdot28+7\cdot35\cdot49}\)

\(=\)\(\frac{1\cdot3\cdot5\cdot\left(1+2+4+7\right)}{1\cdot5\cdot7\cdot\left(1+2+7+7\right)}\)

\(=\frac{1\cdot3\cdot5}{1\cdot5\cdot7}\)\(=\frac{15}{35}=\frac{3}{7}\)

bn vội quá viết nhầm lun kìa 

hj hj chúc bn làm bài tốt nha

1/10+2/20+3/30+4/40+5/50+6/60+7/70+8/80+9/90

=1/10+1/10+1/10+1/10+1/10+1/10+1/10+1/10+1/10

=1/10*9=9/10

9/10 nha

=1/10+1/10+1/10+1/10+1/10+1/10+1/10+1/10+1/10

=1/10x9

=9/10

`1/10+2/20+3/30+4/40+5/50+6/60+7/70+8/80+9/90`

`=1/10+1/10+1/10+1/10+1/10+1/10+1/10+1/10+1/10`

`=1/10xx9`

`=9/10`

giúp mình bài này nha

a,đề bài

=1/10+1/10+1/10+1/10+1/10+1/10+1/10+1/10+1/10

=9/10

Phép tính:1/10+1/10+1/10+1/10+1/10+1/10+1/10+1/10+1/10 là rút gọn lần lượt cho 2,3,4,5,6,7,8,9 nha

\(\frac{1}{10}+\frac{2}{20}+\frac{3}{30}+\frac{4}{40}+\frac{5}{50}+\frac{6}{60}+\frac{7}{70}+\frac{8}{80}+\frac{9}{90}\)

=\(\frac{1}{10}+\frac{1}{10}+\frac{1}{10}+\frac{1}{10}+\frac{1}{10}+\frac{1}{10}+\frac{1}{10}+\frac{1}{10}+\frac{1}{10}\)

=\(\frac{9}{10}\)

\(\frac{1}{10}+\frac{2}{20}+\frac{3}{30}+\frac{4}{40}+\frac{5}{50}+\frac{6}{60}+\frac{7}{70}+\frac{8}{80}+\frac{9}{90}=\frac{1}{10}+\frac{1}{10}+....+\frac{1}{10}=\frac{9}{10}\)

\(\frac{2}{3}\times\frac{4}{5}+\frac{4}{5}\times\frac{1}{2}+\frac{4}{5}\times\frac{1}{6}\)

\(=\left(\frac{2}{3}+\frac{1}{2}+\frac{1}{6}\right)\times\frac{4}{5}\)

\(=\frac{4}{3}\times\frac{4}{5}\)

\(=\frac{16}{15}\)

90:5+110:5=(90+110):5

                 =200:5

                 =40

HT

90:5+110:5                                 90:5+110:5

= (90+110) : 5                            = 18 + 22

= 200         : 5                            =  40

= 40

\(\frac{3}{4}x\frac{4}{5}:\frac{3}{5}x\frac{6}{7}:\frac{1}{7}x\frac{5}{6}\)\(=\)\(\frac{3}{4}x\frac{4}{5}x\frac{5}{3}x\frac{6}{7}x\frac{7}{1}x\frac{5}{6}\)\(=\)\(\frac{3x4x5x6x7x5}{4x5x3x7x1x6}\)\(=\)\(\frac{\left(3\right)x\left(4\right)x\left(5\right)x\left(6\right)x\left(7\right)x5}{\left(3\right)x\left(4\right)x\left(5\right)x\left(6\right)x\left(7\right)x1}\)\(=\)\(\frac{5}{1}\)\(=\)\(5\).

5