= (2/5 + 3/5) + (6/9 + 1/3) + (7/4 + 1/4)

= 1 + (6/9 + 3/9) + 2 = 1 + 1 + 2 = 4

\(\dfrac{2}{5}+\dfrac{6}{9}+\dfrac{7}{4}+\dfrac{3}{5}+\dfrac{1}{3}+\dfrac{1}{4}\\ =\left(\dfrac{2}{5}+\dfrac{3}{5}\right)+\left(\dfrac{7}{4}+\dfrac{1}{4}\right)+\left(\dfrac{2}{3}+\dfrac{1}{3}\right)\\ =1+2+1=4\)

\(\dfrac{2}{5}+\dfrac{6}{9}+\dfrac{7}{4}+\dfrac{3}{5}+\dfrac{1}{3}+\dfrac{1}{4}\)

=\(\dfrac{2}{5}+\dfrac{2}{3}+\dfrac{7}{4}+\dfrac{3}{5}+\dfrac{1}{3}+\dfrac{1}{4}\)

=\(\left(\dfrac{2}{5}+\dfrac{3}{5}\right)+ \left(\dfrac{2}{3}+\dfrac{1}{3}\right)+\left(\dfrac{7}{4}+\dfrac{1}{4}\right)\)

=1+1+2

=4

a)  -187 + (-1907) - ( 1706 - 619 - 581 )

=  -187 + (-1907) - 1706 + 619 + 581

= -187 +(-1907) + (-1706) + 619 + 581

= [-187 + (-1907) + (-1706)] + (619 +581)

= -3800 + 1200

= -2600

b)   1 - 2 - 3 + 4 + 5 - 6 - 7 + 8 +.....+ 2013 - 2014 - 2015 +2016

= (1-2-3+4) + (5-6-7+8) + .............+ (2013-2014-2015+2016)

=0 + 0 +.........+0

=0

a)  -187 + (-1907) - ( 1706 - 619 - 581 )

=-187 + (-1907) -1706 + 619 + 581

=........

=.....

=-3800 + 1200

=-2600

b) 0

a) -187 + (-1907) - ( 1706 - 619 - 581 )

= -187 - 1907 - 1706 + 619 + 581

= ( -187 - 1907 - 1706 ) + ( 619 + 581 )

= -3800 + 1200

= -2600

b) 1 - 2 - 3 + 4 + 5 - 6 - 7 + 8 + ....... + 2013 - 2014 - 2015 + 2016

= ( 1 - 2 - 3 + 4 ) + ( 5 - 6 - 7 + 8 ) + ..... + ( 2013 - 2014 - 2015 + 2016 )

= 0 + 0 + ..... + 0 ( có tất cả 504 số 0 )

= 0 . 504

= 0

chúc bạn học tốt ^_^ 

13-12+11+10-9+8-7-6+5-4+3+2-1
=13-(12+1) -(11+2)-(10+3)-(9+4)-(8+5) +(7+6)
=13-13-13-13-13-13+13
=0-0-0-0-0-0+13
=0+13
=13

sửa lại đề bài đi : Tính hợp lí:13 - 12 + 11 + 10 - 9 + 8 - 7 - 6 + 5 - 4 + 3 +2 - 1

:A

a. \(\dfrac{-2}{3}+\dfrac{-1}{5}+\dfrac{3}{4}-\dfrac{5}{6}-\dfrac{7}{10}\)

= \(\dfrac{-4}{6}+\dfrac{-2}{10}+\dfrac{3}{4}-\dfrac{5}{6}-\dfrac{7}{10}\)

= \(\dfrac{-3}{2}+\dfrac{1}{2}+\dfrac{3}{4}\)

= (-1) + \(\dfrac{3}{4}\)

= \(\dfrac{-4}{4}+\dfrac{3}{4}\)

= \(\dfrac{-1}{4}\)

b; 0,5  + \(\dfrac{1}{3}\) + 0,4 + \(\dfrac{5}{7}\) + \(\dfrac{1}{6}\) - \(\dfrac{4}{35}\)

= (\(\dfrac{1}{3}\)+ \(\dfrac{1}{6}\) + \(\dfrac{1}{2}\)) + (\(\dfrac{5}{7}\)- \(\dfrac{4}{35}\)+ \(\dfrac{2}{5}\))

= ( \(\dfrac{1}{2}\) + \(\dfrac{1}{2}\)) + (\(\dfrac{3}{5}\) + \(\dfrac{2}{5}\))

= 1 + 1 

= 2

c; \(\dfrac{1}{3}\) + \(\dfrac{-1}{5}\) - \(\dfrac{-1}{4}\) - \(\dfrac{7}{9}\)- \(\dfrac{-5}{12}\) + \(\dfrac{-1}{45}\) + \(\dfrac{1}{41}\)

  = (\(\dfrac{1}{3}\) + \(\dfrac{1}{4}\) + \(\dfrac{5}{12}\)) - (\(\dfrac{1}{5}\) + \(\dfrac{7}{9}\) + \(\dfrac{1}{45}\)) + \(\dfrac{1}{41}\)

= (\(\dfrac{4}{12}\) + \(\dfrac{3}{12}\) + \(\dfrac{5}{12}\)) - (\(\dfrac{9}{45}+\dfrac{35}{45}\) + \(\dfrac{1}{45}\)) + \(\dfrac{1}{41}\)

= (\(\dfrac{7}{12}\) + \(\dfrac{5}{12}\)) - (\(\dfrac{44}{45}\) + \(\dfrac{1}{45}\)) + \(\dfrac{1}{41}\)

= 1 - 1 + \(\dfrac{1}{41}\)

= 0 + \(\dfrac{1}{41}\)

= \(\dfrac{1}{41}\)

`a, 4/9 + 2/5 + 5/9= (4/9 +5/9) + 2/5= 1 + 2/5= 5/5 + 2/5= 7/5`

`b, 6/13 + 5/8 + 7/13=(6/13 + 7/13) + 5/8=1+5/8= 8/8 + 5/8= 13/8`

`c, 5/11 + 6/25 + 66/121 = ( 5/11 + 66/121 ) + 6/25=1 + 6/25 = 25/25 + 6/25 = 31/25`

`d, 1/2 + 1/3 + 1/4 + 1/6 + 1/12 + 1/24= (1/2 + 1/4) +(1/3 + 1/6) + (1/12 + 1/24)= 3/4 + 1/2 + 1/8=  6/8 + 4/8 + 1/8= 11/8`

`@ yl`

`a: =(7/3-4/3)+11/5=16/5`

`b: =(18/13-5/13)+55/46=1+55/46=101/46`

`c: =27/25-4/9-2/25+5/9=(27/25-2/25)+(5/9-4/9)=1+1/9=10/9`

1: =5/8*17/13*7/17*8/3*6

=5/3*7/13*6

=5*7*2/13=70/13

2: =-7/9(24/19-1)

=-7/9*5/19

=-35/171

3: =-3/8(7/6-9/6)

=-3/8*(-2/6)

=3/8*1/3=1/8

mk thấy các số trong B đều là số lẻ, nhưng có mỗi số cuối là số chẵn, nên chắc bn ghi nhầm rùi mk sửa :) nếu đề vx như z thì nói mk , mk làm lại cho

...

B = 1 + 3 - 5 -7 + 9 + 11 - 13 - ... - 2013 - 2015 + 2017

B = 1 + (3-5-7+9) + (11-13-15+17)+...+ (2011-2013-2015+2017)

B = 1 + 0 + 0 + ....+ 0

B = 1

Đề đúng bn nha ((:

Mk làm lại cho các bn tham khảo :>>>

B = 1 + 3 - 5 - 7 + 9 + 11 - 13 - ... - 2013 - 2015 + 2016

Từ 1 đến 2015 có: ( 2015 - 1 ) : 2 + 1 = 1008 ( số hạng )

B = ( 1 + 3 - 5 - 7 ) + ( 9 + 11 - 13 - 15 ) + .....+ ( 2009 + 2011 - 2013 - 2015 ) + 2016    ( 252 cặp )

B =  -8 + ( -8 ) + ... + ( -8 ) + 2016  ( 252 số hạng )

B = -8 . 252 + 2016

B = -2016 + 2016

B = 0

-Làm cách của bn ßا§™ cx đc 

-Mk còn cách khác nữa :33

B = 1 + 3 - 5 - 7 + 9 + 11 - 13 - ... - 2013 - 2015 + 2016 

Từ 1 -> 2015 có: ( 2015 - 1 ) : 2 + 1 = 1008

Ta ghép 2 số 1 cặp => Có 504 cặp

B = ( 1 - 5 ) + ( 3 - 7 ) + ( 9 - 13 ) + ( 11 - 15 ) + .... + (  2009 - 2013 ) + ( 2011 - 2015 ) + 2016

B = -4 + ( -4 ) + ( -4 ) + ( -4 ) + ... + ( -4 ) + ( -4 ) + 2016 

504 cặp => 504 số

B = -4 . 504 + 2016

B = -2016 + 2016 

B = 0