a, 3/7 .9/26 -1/14 .1/13 -`1/7

= 27/182 -1/182 -1/7

= 1/7 -1/7

= 0

b,[2/3 -1/4 +5/11]:[5/12+1-7/12]

= 115/132 : 103/132

= 115/103

C; -1/21 +-1/28--1/21-3/14=-1/4

d,[-1/5+3/12] +-3/4

=1/20 +-3/4

=-7/10

\(\dfrac{x}{7}-x=\dfrac{1}{21}\)

=> \(\dfrac{x-7x}{7}=\dfrac{1}{21}\)

=> \(\dfrac{-6x}{7}=\dfrac{1}{21}\)

=> -6x.21=7

=> -126.x=7

=> x=\(\dfrac{7}{-126}=\dfrac{-1}{18}\)

\(\dfrac{x}{7}-x=\dfrac{1}{21}\Leftrightarrow\left(\dfrac{1}{7}-1\right)x=\dfrac{1}{21}\Leftrightarrow\dfrac{-6}{7}x=\dfrac{1}{21}\)

\(x=\dfrac{-6.21}{7}=-6.3=-18\)

\(\dfrac{x}{7}-x=\dfrac{1}{21}\Leftrightarrow\left(\dfrac{1}{7}-1\right)x=\dfrac{1}{21}\Leftrightarrow x=\dfrac{\dfrac{1}{21}}{\dfrac{-6}{7}}=\dfrac{1}{21}.\left(-\dfrac{7}{6}\right)=-\dfrac{1}{18}\)

\(-\dfrac{4}{7}-x=\dfrac{3}{5}-2x\\ \Rightarrow-x+2x=\dfrac{3}{5}+\dfrac{4}{7}\\ \Rightarrow x=\dfrac{21}{35}+\dfrac{20}{35}\\ \Rightarrow x=\dfrac{41}{35}\)

Vậy `x=41/35`

__

\(\dfrac{3}{7}x-\dfrac{2}{3}x=\dfrac{10}{21}\\ \Rightarrow\left(\dfrac{3}{7}-\dfrac{2}{3}\right)x=\dfrac{10}{21}\\ \Rightarrow\left(\dfrac{9}{21}-\dfrac{14}{21}\right)x=\dfrac{10}{21}\\ \Rightarrow\dfrac{-5}{21}x=\dfrac{10}{21}\\ \Rightarrow x=\dfrac{10}{21}:\left(-\dfrac{5}{21}\right)\\ \Rightarrow x=-2\)

Vậy `x=-2`

a)

-4/7 - x = 3/5 - 2x

2x - x = 3/5 + 4/7

x = 41/35

Vậy x = 41/35

b)

3/7.x - 2/3.x = 10/21

x(3/7 - 2/3) = 10/21

x.(-5/21) = 10/21

x = 10/21 : (-5/21) = -2

Vậy x = -2

=> B/2 = 1/6 + 1/12 + 1/20 +...+ 1/90

= 1/2.3 + 1/3.4 + 1/4.5 +... +1/9.10

=1/2 - 1/3 + 1/3 -...+ 1/9 -1/10

=1/2-1/10

=> B = 1-1/5 = 4/5

a: =1/8-1/8+2/5=2/5

b: =(-1/15-14/15)+(23/27+31/27)=2-1=1

c: \(=\dfrac{3}{7}\left(\dfrac{22}{21}+\dfrac{5}{21}+\dfrac{15}{21}\right)=\dfrac{3}{7}\cdot2=\dfrac{6}{7}\)

d: \(=\dfrac{-8}{9}\cdot\dfrac{3}{2}+\dfrac{1}{9}\cdot\dfrac{-3}{2}=-\dfrac{3}{2}\)

\(=\left[\dfrac{\sqrt{7}-1}{\sqrt{3}\left(\sqrt{7}-1\right)}+\dfrac{\sqrt{3}-1}{\sqrt{7}\left(\sqrt{3}-1\right)}\right]\cdot\dfrac{\sqrt{21}}{\sqrt{7}+\sqrt{3}}\\ =\left(\dfrac{1}{\sqrt{3}}+\dfrac{1}{\sqrt{7}}\right)\cdot\dfrac{\sqrt{21}}{\sqrt{7}+\sqrt{3}}\\ =\dfrac{\sqrt{3}+\sqrt{7}}{\sqrt{21}}\cdot\dfrac{\sqrt{21}}{\sqrt{3}+\sqrt{7}}=1\)

bấm máy tính ra kết quả là 983/504

Lời giải:
$B=\frac{3}{2}-\frac{2}{21}-\frac{7}{12}+\frac{15}{21}-\frac{1}{3}+\frac{5}{4}-\frac{2}{7}-\frac{1}{3}$

$=(\frac{3}{2}+\frac{5}{4})+(\frac{-2}{21}+\frac{15}{21}+\frac{-2}{7})+(\frac{-7}{12}+\frac{-1}{3}+\frac{-1}{3})$

$=\frac{11}{4}+\frac{1}{3}-\frac{5}{4}=\frac{11}{6}$