(8 x 7 - 16 - 5 x 8) x (1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9)

= [8 x 7 - 8 x 2 - 5 x 8] x [(1 + 9) + (2 + 8 ) + (3 + 7) + (4 + 6) + 5]

= [8 x (7 - 2 - 5)] x [10 + 10 + 10 + 10 + 10 + 5]

= (8 x 0) x 45

= 0

1 + 3 + 5 + 7 + 9 + 11 + 13 + 15 + 17 + 19

= (1 + 9) + (3 + 7) + (5 + 15) + (11 + 19) + (13 + 17)

= 10 + 10 + 20 + 30 + 30

= 100

26 giờ 35 phút

=26 giờ 35 phút

a,Ta có :

1/3 + 1/16 + 1/19 + 1/21 + 1/61 + 1/72 + 1/83 + 1/94 = 0,54

3/4 = 0,6

=>b < 3/4

a)2

b)75/37

c)0

d)8/27

e)7/19

f)4

ok

a) \(\frac{2}{9}+\frac{1}{5}+\frac{7}{9}+\frac{4}{5}=\left(\frac{2}{9}+\frac{7}{9}\right)+\left(\frac{1}{5}+\frac{4}{5}\right)= 1+1=2 \)

b) \(\frac{19}{37}+\left(1+\frac{19}{37}\right)=\left(\frac{19}{37}-\frac{19}{37}\right)+1=0+1=1\)

c) \(\frac{9}{8}-\left(\frac{17}{7}-\frac{3}{7}\right)+\frac{7}{8}=\frac{9}{8}+\frac{7}{8}-\left(\frac{17}{7}-\frac{3}{7}\right)=2-2=0\)

d) \(\frac{3}{5}\times\frac{8}{27}\times\frac{5}{3}=\frac{3}{5}\times\frac{5}{3}\times\frac{8}{27}=1\times\frac{8}{27}=\frac{8}{27}\)

e) \(\frac{7}{19}\times\frac{1}{3}+\frac{7}{19}\times\frac{2}{3}=\frac{7}{19}\times\left(\frac{1}{3}+\frac{2}{3}\right)=\frac{7}{19}\times1=\frac{7}{19}\)

f) \(\frac{12}{5}\times4-4\times\frac{7}{5}=4\times\left(\frac{12}{5}-\frac{7}{5}\right)=4\times1=4\)

\(\dfrac{2}{3}\times\dfrac{3}{4}+\dfrac{3}{4}\times\dfrac{2}{5}=\dfrac{3}{4}\times\left(\dfrac{2}{3}+\dfrac{2}{5}\right)=\dfrac{3}{4}\times\dfrac{16}{15}=\dfrac{4}{5}\)

0.8

0,8

\(A=\frac{1}{3\times5}+\frac{3}{5\times11}+\frac{2}{11\times15}+\frac{3}{15\times21}\)

\(A=\frac{1}{2}\times\left(\frac{2}{3\times5}+\frac{6}{5\times11}+\frac{4}{11\times15}+\frac{6}{15\times21}\right)\)

\(A=\frac{1}{2}\times\left(\frac{1}{3}-\frac{1}{5}+\frac{1}{5}-\frac{1}{11}+\frac{1}{11}-\frac{1}{15}+\frac{1}{15}-\frac{1}{21}\right)\)

\(A=\frac{1}{2}\times\left(\frac{1}{3}-\frac{1}{21}\right)\)

\(A=\frac{1}{2}\times\frac{2}{7}\)

\(A=\frac{1}{7}\)

A = \(\frac{1}{3.5}+\frac{3}{5.11}+\frac{2}{11.15}+\frac{3}{15.21}\)

A = \(\frac{1}{2}.\left(\frac{2}{3.5}+\frac{6}{5.11}+\frac{4}{11.15}+\frac{6}{15.21}\right)\)

A = \(\frac{1}{2}.\left(\frac{1}{3}-\frac{1}{5}+\frac{1}{5}-\frac{1}{11}+\frac{1}{11}-\frac{1}{15}+\frac{1}{15}-\frac{1}{21}\right)\)

A = \(\frac{1}{2}.\left(\frac{1}{3}-\frac{1}{21}\right)\)

A = \(\frac{1}{2}.\frac{2}{7}\)

A = \(\frac{1}{7}\)

2/19 x 3/7 + 3/7 x 4/19 + 13/19 x 3/7 

= ( 2/19 + 4/19 + 13/19) x 3/7

= 19/19  x 3/7

= 1 x 3/7

= 3/7

=3/7.(2/19+4/19+13/19)

=3/7.1

=3/7

`2/19 xx 3/7 + 3/7 xx 4/19 + 13/19 xx 3/7`

`= ( 2/19 + 4/19 + 13/19) xx 3/7`

`= 19/19  xx 3/7`

`= 1 xx 3/7`

`= 3/7`

1) 123. 456456-456.123123

= 123. 123123.2 - 123. 123123.2

= 0

2) 127.313313-13.127127

= 127. 313.1001 - 13. 1001.127

= 127127.313 - 13.127127

= 127127. ( 313-13) 

=127127.300 = 38138100

3) (98.7676-9898.76).(1+2+3+......+100)

= (98. 101.76 - 98.101.76).(1+2+3+......+100)

= 0 .(1+2+3+........+100)

=0

4) 19 + 19 +......+19+77+77+....+77

= 19x 23 + 77 x19 

= 19 x ( 23+77) 

= 19 x 100

= 1900 

\(\frac{2}{1.2}+\frac{2}{2.3}+\frac{2}{3.4}+....+\frac{2}{18.19}+\frac{2}{19.20}\)

\(=2\cdot\left(1-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+...+\frac{1}{18}-\frac{1}{19}+\frac{1}{19}-\frac{1}{20}\right)\)

\(=2\cdot\left(1-\frac{1}{20}\right)\)

\(=2\cdot\frac{19}{20}=\frac{19}{10}\)