( 764 - 759 ) + ( 368 - 369 ) 

= 5 - 5 = 10 

b) ( 4172 - 4168 ) - (1873 - 1869 ) 

= 4-4 = 0

ko biết nữa

là sao

=> 203=56+x

=>x=203-56

=>x=147

x=147

\(\)\(\dfrac{10}{56}+\dfrac{10}{140}+...+\dfrac{10}{1400}\)

\(=\dfrac{5}{28}+\dfrac{5}{70}+...+\dfrac{5}{700}\)

\(=\dfrac{5}{3}\left(\dfrac{1}{4}-\dfrac{1}{7}+\dfrac{1}{7}-\dfrac{1}{10}+...+\dfrac{1}{25}-\dfrac{1}{28}\right)\)

\(=\dfrac{5}{3}\left(\dfrac{1}{4}-\dfrac{1}{28}\right)\)

\(=\dfrac{5}{3}\cdot\dfrac{6}{28}=2\cdot\dfrac{5}{28}=\dfrac{10}{28}=\dfrac{5}{14}\)

\(=\dfrac{5}{28}+\dfrac{5}{70}+\dfrac{5}{130}+...+\dfrac{5}{700}\\ =\dfrac{5}{3}\left(\dfrac{3}{4\cdot7}+\dfrac{3}{7\cdot10}+\dfrac{3}{10\cdot13}+...+\dfrac{3}{25\cdot28}\right)\\ =\dfrac{5}{3}\left(\dfrac{1}{4}-\dfrac{1}{7}+\dfrac{1}{7}-\dfrac{1}{10}+...+\dfrac{1}{25}-\dfrac{1}{28}\right)\\ =\dfrac{5}{3}\left(\dfrac{1}{4}-\dfrac{1}{28}\right)=\dfrac{5}{3}\cdot\dfrac{3}{14}=\dfrac{5}{14}\)

\(\left(-57\right).\left(-46\right)+57.\left(-56\right)\)

\(=\left(-57\right).\left(-46+56\right)\)

\(=\left(-57\right).\left(10\right)\)

\(=-570\)

a

a

a

a

a

(-57).(-46)+57.(-56)

=57.46+57.(-56)

=57.[46+(-56)]

=57.(-10)

=-570

Rút gọn rồi tách mẫu là đ