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

`2/3 xx 4/9 + 2/3 xx 5/9 `

`= 2/3 xx (4/9 + 5/9)`

`= 2/3 xx 1`

`= 2/3`

1 + 9 + 2 + 8 + 3 + 7 + 4 + 6 + 5 + 5 + 6 + 4 + 7 + 3 + 8 + 2 + 9 + 1

= 10 + 10 + 10 +10 +10 + 10 + 10 + 10 + 10

= 10 x 9

= 90

1+9+2+8+3+7+4+6+5+5+6+4+7+3+8+2+9+1

=10+10+10+10+10

=50

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

\(12\times\dfrac{3}{4}-\dfrac{3}{4}\times\dfrac{12}{7}=\dfrac{3}{4}\times\left(12-\dfrac{12}{7}\right)=\dfrac{3}{4}\times\dfrac{72}{7}=\dfrac{54}{7}\)

Đặt S = 1 x 2 + 2 x 3 + ..... + 9 x 10

3S = 1 x 2 x (3-0) + 2 x 3 x (4 - 1) + ....... + 9 x 10 x (11 - 8)

3S = 1 x 2 x 3 + 2 x 3 x 4 - 1 x 2 x 3 + ......... + 9 x 10 x 11 - 8 x 9 x 10

3S = ( 1 x 2 x 3 -1 x 2 x 3) + ( 2 x 3 x4 - 2 x 3 x 4) + ..... + (8 x 9  x 10 - 8 x 9 x 10) + 9 x 10 x 11

3S = 9 x 10 x 11 = 990

S = 990 : 3 = 330     

a,19/18    b,27/40

có sai đề không vậy

minh ko co sai de boi vi cai nay minh tu dat ra ma

minh ghi sai de roi

Bài làm

- \(\frac{4}{5}+\frac{3}{7}\)

= \(\frac{28}{35}+\frac{15}{35}=\frac{43}{35}\)

\(\frac{4}{5}+\frac{3}{7}=\frac{43}{35}\)
\(\frac{5}{6}-\frac{1}{3}=\frac{1}{2}\)
\(\frac{4}{9}.3=\frac{4}{3}\)
\(\frac{1}{2}:\frac{3}{2}=\frac{1}{3}\)