15 x 7 105         30 x 6 180         28 x 7 196         42 x 5 210

a. 2.54

b. 0.5

=\(\dfrac{2}{3}\times\left(\dfrac{4}{9}+\dfrac{5}{9}\right)=\dfrac{2}{3}\times\dfrac{9}{9}=\dfrac{2}{3}\)

\(=\dfrac{2}{3}\times\left(\dfrac{4}{9}+\dfrac{5}{9}\right)\\ =\dfrac{2}{3}\times1=\dfrac{2}{3}\)

\(=\dfrac{2}{3}\left(\dfrac{4}{9}+\dfrac{5}{9}\right)=\dfrac{2}{3}\)

2/5

2/5

= 2/5 x ( 8/7 - 1/7)

= 2/5 x 1

= 2/5

\(\dfrac{2}{15}\times\dfrac{1}{8}\times\dfrac{15}{12}=1\times\dfrac{1}{8}\times\dfrac{1}{6}=\dfrac{1}{48}\)

=(2/15x15/12)x1/8

= 1/6x 1/8

= 1/48

( nếu có sai thì thui nha)

1/48

\(\dfrac{2}{9}+\dfrac{3}{7}+\dfrac{15}{13}+\dfrac{4}{7}-\dfrac{2}{13}+\dfrac{7}{9}\)

\(=\left(\dfrac{2}{9}+\dfrac{7}{9}\right)+\left(\dfrac{3}{7}+\dfrac{4}{7}\right)+\left(\dfrac{15}{13}-\dfrac{2}{13}\right)\)

\(=\dfrac{9}{9}+\dfrac{7}{7}+\dfrac{13}{13}=1+1+1=3\)

`2/9+3/7+15/13+4/7-2/13+7/9`

`= (2/9 + 7/9 ) + ( 3/7 + 4/7 ) + ( 15/13-2/13 ) `

`= 9/9 + 7/7 + 13/13`

`=   1 + 1 + 1 = 3`

a: =11,2x100=1120

b: =3,45x10=34,5

a, =11,2×(36+63+1)

=11,2×100=1120

b, =3,45×(18,5-2,3-6,2)

=3,45×10=34,5

=2+8/7-2/7

=2+6/7

=20/7

14,5 x  2,7 + 2,7  x 5,5 - 2,7 x  2  x 5.

= 2,7 x 14,5 + 2,7 x 5,5 - 2,7 x 10

= 2,7 x ( 14,5 + 5,5 - 10 )

= 2,7 x 10

= 27

\(a;\frac{3}{7}+\frac{2}{3}=\frac{9}{21}+\frac{14}{21}=\frac{23}{21}\)

\(b;\frac{8}{7}-\frac{12}{28}\times\frac{7}{3}=\frac{8}{7}-\frac{3}{7}\times\frac{7}{3}=\frac{8}{7}-1=\frac{8}{7}-\frac{7}{7}=\frac{1}{7}\)

\(c;\frac{5}{6}\times3=\frac{15}{6}=\frac{5}{2}\)

\(d;\frac{3}{2}:\frac{7}{15}=\frac{3}{2}\times\frac{15}{7}=\frac{45}{14}\)

a) 3/7+2/3=9/21+14/21=23/21

b) 8/7-12/28*7/3=8/7-1=8/7-7/7-1/7

c) 5/6*3=5/2

d) 3/2:7/15=3/2*15/7=45/14