63.(56-21)+21.(56-63)

=63.56-63.21+21.56-21.63

=21.3.56-21.3.21+21.56-21.63

=21.168-21.63+21.56-21.63

=21.(168-63)+21.(56-63)

=21.(168-63+56-63)

=21.98

=2058

Chúc bạn học tốt

\(\dfrac{56}{35}\times\dfrac{63}{36}\times\dfrac{6}{9}\times\dfrac{5}{8}=\dfrac{8}{5}\times\dfrac{7}{4}\times\dfrac{2}{3}\times\dfrac{5}{8}=\left(\dfrac{8}{5}\times\dfrac{5}{8}\right)\times\left(\dfrac{7}{4}\times\dfrac{2}{3}\right)=1\times\dfrac{7}{6}=\dfrac{7}{6}\)

\(\dfrac{56}{35}\times\dfrac{63}{36}\times\dfrac{6}{9}\times\dfrac{5}{8}=\dfrac{8}{5}\times\dfrac{7}{4}\times\dfrac{2}{3}\times\dfrac{5}{8}=\left(\dfrac{8}{5}\times\dfrac{5}{8}\right)\times\left(\dfrac{7}{4}\times\dfrac{2}{3}\right)=1\times\dfrac{7}{6}=\dfrac{7}{6}\)

a) (350+1050):14=1400:14

                               =100

b) (1224 x 72):(12 x 8)=88128 : 96

                                  =918

c) 918 x (1715 : 35)=918 x 49

                              =44982

\(\dfrac{-3}{5}+\dfrac{28}{5}.\left(\dfrac{43}{56}+\dfrac{5}{24}-\dfrac{21}{63}\right)\)

= \(\dfrac{-3}{5}+\dfrac{28}{5}.\dfrac{9}{14}\)

= \(\dfrac{-3}{5}+\dfrac{28.9}{5.14}\)

= \(\dfrac{-3}{5}+\dfrac{18}{5}\)

= \(\dfrac{\left(-3\right)+18}{5}\)

= \(\dfrac{15}{5}=3\)

rút gọn thành

1/2 x 2/3 x 3/4 x 4/5 x 5/6 x 6/7 x 7/8 x 8/9

1x2x3x4x5x6x7x8

__________________

2x3x4x5x6x7x8x9

gạch các số giống nhau ở tử và mẫu và bằng 1/9

A = 1/2 + 1/3 + 1/6 + 1/12 + 1/15 + 1/20 + 1/30 + 1/35 + 1/42 + 1/56 + 1/63 + 1/72 + 1/99

   = ( 1/2 + 1/6 + 1/12 + 1/20 + 1/30 + 1/42 + 1/56 + 1/72 ) + ( 1/3 + 1/15 + 1/35 + 1/63 + 1/99 )

   = ( 1/1.2 + 1/2.3 + 1/3.4 + 1/4.5 + ... + 1/8.9 ) + ( 1/1.3 + 1/3.5 + ... + 1/9.11 )

   = ( 1 - 1/2 + 1/2 - 1/3 + ... + 1/8 - 1/9 ) + 1/2 . ( 2/1.3 + 2/3.5 + ... + 2/9.11 )

   = ( 1 - 1/9 ) + 1/2 . ( 1 - 1/3 + 1/3 - 1.5 + ... + 1/9 - 1/11 )

   = 8/9 + 1/2 . ( 1 - 1/11 )

   = 8/9 + 1/2 . 10/11

   = 8/9 + 5/11

   = 133/99

A = 1/2 + 1/3 + 1/6 + 1/12 + 1/15 + 1/20 + 1/30 + 1/35 + 1/42 + 1/56 + 1/63 + 1/72 + 1/99

   = ( 1/2 + 1/6 + 1/12 + 1/20 + 1/30 + 1/42 + 1/56 + 1/72 ) + ( 1/3 + 1/15 + 1/35 + 1/63 + 1/99 )

   = ( 1/1.2 + 1/2.3 + 1/3.4 + 1/4.5 + ... + 1/8.9 ) + ( 1/1.3 + 1/3.5 + ... + 1/9.11 )

   = ( 1 - 1/2 + 1/2 - 1/3 + ... + 1/8 - 1/9 ) + 1/2 . ( 2/1.3 + 2/3.5 + ... + 2/9.11 )

   = ( 1 - 1/9 ) + 1/2 . ( 1 - 1/3 + 1/3 - 1.5 + ... + 1/9 - 1/11 )

   = 8/9 + 1/2 . ( 1 - 1/11 )

   = 8/9 + 1/2 . 10/11

   = 8/9 + 5/11

   = 133/99

A = 1/2 + 1/3 + 1/6 + 1/12 + 1/15 + 1/20 + 1/30 + 1/35 + 1/42 + 1/56 + 1/63 + 1/72 + 1/99

   = ( 1/2 + 1/6 + 1/12 + 1/20 + 1/30 + 1/42 + 1/56 + 1/72 ) + ( 1/3 + 1/15 + 1/35 + 1/63 + 1/99 )

   = ( 1/1.2 + 1/2.3 + 1/3.4 + 1/4.5 + ... + 1/8.9 ) + ( 1/1.3 + 1/3.5 + ... + 1/9.11 )

   = ( 1 - 1/2 + 1/2 - 1/3 + ... + 1/8 - 1/9 ) + 1/2 . ( 2/1.3 + 2/3.5 + ... + 2/9.11 )

   = ( 1 - 1/9 ) + 1/2 . ( 1 - 1/3 + 1/3 - 1.5 + ... + 1/9 - 1/11 )

   = 8/9 + 1/2 . ( 1 - 1/11 )

   = 8/9 + 1/2 . 10/11

   = 8/9 + 5/11

   = 133/99

\(\frac{3}{7}\cdot\frac{56}{63}+\frac{3}{7}\cdot\frac{17}{63}\)

\(=\frac{3}{7}\cdot(\frac{56}{63}+\frac{17}{63})\)

\(=\frac{3}{7}\cdot\frac{73}{63}=\frac{3\cdot73}{7\cdot63}=....\)

 Bạn tự tính nhé

  \(\frac{3}{7}\)x \(\frac{56}{63}\) + \(\frac{3}{7}\)x \(\frac{17}{63}\)

= \(\frac{3}{7}\)x \(\left(\frac{56}{63}+\frac{17}{63}\right)\)

= \(\frac{3}{7}\)x   \(\frac{73}{63}\)

= \(\frac{73}{147}\)

ta có: 2005 + 53-54-55+56-57-58-59+60+61-62-63+64+65-66-67+68+69

      = 2005+(53-54-55+56)+(57-58-59+60)+(61-62-63+64)+(65-66-67+68)+69

      = 2005+0+0+0+0+69

      = 2005+69

      = 2074