B = 2/5x7 + 3/7x10 + 4/10x14 + 7/14x21 + 9/21x30

B = 1/5 - 1/7 + 1/7 - 1/10 + 1/10 - 1/14 + 1/14 - 1/21 + 1/21 - 1/30

B = 1/5 - 1/30

B = 6/30 - 1/30

B = 5/30 = 1/6

B = 2/5x7 + 3/7x10 + 4/10x14 + 7/14x21 + 9/21x30

B = 1/5 - 1/7 + 1/7 - 1/10 + 1/10 - 1/14 + 1/14 - 1/21 + 1/21 - 1/30

B = 1/5 - 1/30

B = 6/30 - 1/30

B = 5/30 = 1/6

soyeon làm sai vì nó là 2/5x7 chứ có phải là 1/5x7 đâu mà lại tách ra như thế?

=(6+0,25)x7,42

=6,25x7,42

=46,375

k mk nha

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

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

\(\dfrac{12}{7}\) \(\times\) \(\dfrac{3}{4}\) - \(\dfrac{3}{4}\) \(\times\) \(\dfrac{12}{7}\)

= \(\dfrac{9}{7}\) - \(\dfrac{9}{7}\)

= 0 

3/2 + 4/9 + 4/7 + 5/9 = 43/14

A = \(2000-\left\{46-\left[24+74.4\right]:15\right\}\)

A = \(2000-\left\{46-\left[24+296\right]:15\right\}\)

A = \(2000-\left\{46-320:15\right\}\)

A = \(2000-\left(46-\frac{64}{3}\right)\)

A = \(2000-\left(\frac{138}{3}-\frac{64}{3}\right)\)

A = \(2000-\frac{74}{3}\)

A = \(\frac{6000}{3}-\frac{74}{3}\)

A = \(\frac{5926}{3}\)