\(A=\frac{-1}{20}+\frac{-1}{30}+\frac{-1}{42}+\frac{-1}{56}+\frac{-1}{72}+\frac{-1}{90}\)

\(A=-\left(\frac{1}{4\cdot5}+\frac{1}{5\cdot6}+\frac{1}{6\cdot7}+\frac{1}{7\cdot8}+\frac{1}{8\cdot9}+\frac{1}{9\cdot10}\right)\)

\(A=-\left(\frac{1}{4}-\frac{1}{5}+\frac{1}{5}-\frac{1}{6}+...+\frac{1}{9}-\frac{1}{10}\right)\)

\(A=-\left(\frac{1}{4}-\frac{1}{10}\right)\)

\(A=\frac{-3}{20}\)

=[(-172)+72]+{56+(-26)
=-100+30
=-70

(-172)+56+(-26)+72 

= [(-172)+72] + [56+(-26)]

=-100+30

=-(100-30)

=-70 

Chúc bn hk tốt!

(-172) + 56 + (-26) + 72

= [(-172) + 72] + [56+(-26)]

= -100 + 30

= -70

(13-135+49)-(13+49)

= 13-135 + 49 - 13-49

= ( 13-13) + ( 49-49) -135

= 0 -135

=-135

26.(24-11)-24.(26-11)

= 26.24 - 11.26 - 24.26 - 11.24

= 24.(26-26) -11(26-24)

= 0 - 11.2

=-22

\(\left(13-135+49\right)-\left(13+49\right)\))

\(=13-135+49-13-49\)

\(=\left(13-13\right)+\left(49-49\right)-135\)

\(=0-135=-135\)

\(26.\left(24-11\right)-24.\left(26-11\right)\)

\(=26.24-11.26-24.26-11.24\)

\(=24.\left(26-26\right)-11.\left(26-24\right)\)

\(=0-11.2=-22\)

(-172)+56+(-26)+72

= ( -172 + 72 ) + ( -26 + 56 )

= -100 + 30

= -70

(-172)+56+(-26)+72

=-172+56-26+72

=(-172+72)+(56-26)

=-100+30

=-70

 a) 136.(-47)+36.47

= -136.47 + 36. 47

=47 . (-136 + 36 )

=47 . (-100)

=-4700

b) (-48).72+36

=(-48).2.36+36

= -96.36+36

=(-96+1).36

= -95.36=-3420

a)136.(-47)+36.47

=47(-136+36)

=47.(-100)

=-4700

b) (-48).72+36

=-3456+36=-3420

15 + 15 x 2 + 15 x 7 - 100

= ( 2 + 7 + 1 ) x 15

= 10 x 15

= 150

15 + 15x2 + 15x7 -100

= 15 x( 1+2+7 ) -100

= 15 x 10 -100

= 150-100

=50

15 + 15x2 + 15x7 - 100

= 15 x (1 + 2 + 7) - 100

= 15 x 10 - 100

= 150 - 100

= 50

\(21\cdot35\cdot3\cdot25\cdot7\)

\(=21\cdot3\cdot7\cdot35\cdot25\)

\(=21(3\cdot7)\cdot35\cdot25\)

\(=21\cdot21\cdot35\cdot25\)

\(=21^2\cdot875=385875\)

Nghĩ sao không có số nào tròn để tính hợp lí -_-

Tổng tren có số số hạng là:

(51 - 1) : 5 + 1 = 11 (số)

Tổng trên là:

(51 + 1) . 11 : 2 = 286

ĐS: 286

35x18-9x70+100

= 35x9x2 - 9x35x2 +100

=  0 +100 = 100

35x18-9x70+100

=35x9x2-9x2x35+100

=0+100

=100

35 x 18 - 9 x 70 + 100

= ( 35 x 9 x 2 ) - ( 9 x 2 x 35 ) + 100

= 0 + 100

= 100