56+57=
1589+1686=
158+260=
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( 764 - 759 ) + ( 368 - 369 )
= 5 - 5 = 10
b) ( 4172 - 4168 ) - (1873 - 1869 )
= 4-4 = 0
\(\)\(\dfrac{10}{56}+\dfrac{10}{140}+...+\dfrac{10}{1400}\)
\(=\dfrac{5}{28}+\dfrac{5}{70}+...+\dfrac{5}{700}\)
\(=\dfrac{5}{3}\left(\dfrac{1}{4}-\dfrac{1}{7}+\dfrac{1}{7}-\dfrac{1}{10}+...+\dfrac{1}{25}-\dfrac{1}{28}\right)\)
\(=\dfrac{5}{3}\left(\dfrac{1}{4}-\dfrac{1}{28}\right)\)
\(=\dfrac{5}{3}\cdot\dfrac{6}{28}=2\cdot\dfrac{5}{28}=\dfrac{10}{28}=\dfrac{5}{14}\)
\(=\dfrac{5}{28}+\dfrac{5}{70}+\dfrac{5}{130}+...+\dfrac{5}{700}\\ =\dfrac{5}{3}\left(\dfrac{3}{4\cdot7}+\dfrac{3}{7\cdot10}+\dfrac{3}{10\cdot13}+...+\dfrac{3}{25\cdot28}\right)\\ =\dfrac{5}{3}\left(\dfrac{1}{4}-\dfrac{1}{7}+\dfrac{1}{7}-\dfrac{1}{10}+...+\dfrac{1}{25}-\dfrac{1}{28}\right)\\ =\dfrac{5}{3}\left(\dfrac{1}{4}-\dfrac{1}{28}\right)=\dfrac{5}{3}\cdot\dfrac{3}{14}=\dfrac{5}{14}\)
\(\left(-57\right).\left(-46\right)+57.\left(-56\right)\)
\(=\left(-57\right).\left(-46+56\right)\)
\(=\left(-57\right).\left(10\right)\)
\(=-570\)
a
a
a
a
a
(-57).(-46)+57.(-56)
=57.46+57.(-56)
=57.[46+(-56)]
=57.(-10)
=-570
113
3275
318
56+57=113
1589+1686=3275
158+260=418
kick mik nhé mọi người