Ta có :

\(A=\frac{1}{3}-\frac{3}{4}-\left(-\frac{3}{5}\right)+\frac{1}{72}-\frac{2}{9}-\frac{1}{36}+\frac{1}{15}\)

\(\Rightarrow A=\frac{5}{15}-\frac{54}{72}+\frac{9}{15}+\frac{1}{72}-\frac{16}{72}-\frac{1}{72}+\frac{1}{15}\)

\(\Rightarrow A=\left(\frac{5}{15}+\frac{9}{15}+\frac{1}{15}\right)+\left(-\frac{54}{72}+\frac{1}{72}-\frac{16}{72}-\frac{2}{72}\right)\)

\(\Rightarrow A=1-\frac{71}{72}=\frac{1}{72}\)

1) 3/9 + 5/9 = 8/9

2) 1/2 x 2/3 x 3/4 x 4/5

= 1/5

(1-1/2)×(1-1/3)×(1-1/4)×(1-1/5)×1-1/6=1/2×2/3×3/4×4/5×5/6=1/6

\(\dfrac{2}{3}:\dfrac{4}{3}:\dfrac{5}{4}:\dfrac{1}{5}\)

=\(\dfrac{2}{3}\cdot\dfrac{3}{4}:\dfrac{5}{4}\cdot\dfrac{5}{1}\)

=\(\dfrac{1}{2}\cdot\dfrac{25}{4}\)

=\(\dfrac{25}{8}\)

\(\dfrac{1}{4}+\dfrac{2}{5}+\dfrac{3}{4}+\dfrac{3}{5}\)
\(=\left(\dfrac{1}{4}+\dfrac{3}{4}\right)+\left(\dfrac{2}{5}+\dfrac{3}{5}\right)\)
\(=1+1\)
\(=2\)

`1/4+2/5+3/4+3/5`

`=(1/4+3/4)+(2/5+3/5)`

`=4/4+5/5`

`=1+1`

`=2`

= ( 1/4 + 3/4 ) + ( 2/5 + 3/5 ) 

= 1 + 1

= 2

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

\(\Leftrightarrow\left(\frac{3}{4}+\frac{1}{4}\right)+\left(\frac{2}{5}+\frac{3}{5}\right)\)

\(=1+1\)

\(=2\)

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

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

\(=\frac{4}{4}+\frac{5}{5}\)

\(=1+1\)

\(=2\)

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

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

\(=1+1\)

\(=2\)

Có số số hạng là:

( 300- 1) : 1+ 1 = 300

Tổng dãy số trên là:

( 300+ 1) x 300 : 2 = 45150

               Đáp số: 45150

\(...=\left(300-1+1\right)\left(1+300\right):2=300.301:2=150.301=45150\)

=\(\frac{2}{5}+1\)

=\(\frac{7}{5}\)

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

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

\(=\frac{2}{5}+1\)

\(=\frac{2}{5}+\frac{5}{5}\)

\(=\frac{7}{5}\)

Trả lời :

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

\(=\frac{2}{5}+1\)

\(=\frac{7}{5}\)

1) \(=\left(\frac{3}{5}+\frac{2}{5}\right).\frac{6}{11}\)

\(=1.\frac{6}{11}\)

\(=\frac{6}{11}\)

2)\(\frac{17}{25}.\left(\frac{11}{19}+\frac{6}{19}+\frac{2}{19}\right)\)

\(=\frac{17}{25}.1\)

\(=\frac{17}{25}\)

2.\(\frac{17}{25}.\frac{11}{19}+\frac{17}{25}.\frac{6}{19}+\frac{17}{25}.\frac{2}{19}=\frac{17}{25}\left(\frac{11}{19}+\frac{6}{19}+\frac{2}{19}\right)\\ =\frac{17}{25}.1\\ =\frac{17}{25}\)

`4/6 xx 1/5 + 4/6 xx 3/5 + 2/6 xx 4/5`

`= 4/6 xx (1/5 + 3/5) + 2/6 xx 4/5`

`= 4/6 xx 4/5 + 2/6 xx 4/5`

`= (4/6 +2/6) xx 4/5`

`= 6/6 xx 4/5`

`=1 xx 4/5`

`=4/5`

Em cảm ơn