A)\(\dfrac{7}{20}-\left(\dfrac{5}{8}-\dfrac{2}{5}\right)\)

\(=\dfrac{7}{20}-\left(\dfrac{25}{40}-\dfrac{16}{40}\right)\)

\(=\dfrac{7}{20}-\dfrac{9}{40}\)

\(=\dfrac{14}{40}-\dfrac{9}{40}=\dfrac{5}{40}=\dfrac{1}{8}\)

B) \(\dfrac{5}{6}+\left(\dfrac{5}{9}-\dfrac{1}{4}\right)\)

\(=\dfrac{5}{6}+\left(\dfrac{20}{36}-\dfrac{9}{36}\right)\)

\(=\dfrac{5}{6}+\dfrac{11}{36}\).

\(=\dfrac{30}{36}+\dfrac{11}{36}=\dfrac{41}{36}\)

C) \(\dfrac{9}{10}-\left(\dfrac{2}{5}-\dfrac{3}{10}\right)+\dfrac{7}{20}\)

\(=\dfrac{9}{10}-\left(\dfrac{4}{10}-\dfrac{3}{10}\right)+\dfrac{7}{20}\)

\(=\dfrac{9}{10}-\dfrac{1}{10}+\dfrac{7}{20}\)

\(=\dfrac{18}{20}-\dfrac{2}{20}+\dfrac{7}{20}=\dfrac{23}{20}\)

a: =7/20-5/8+2/5

=14/40-25/40+16/40

=5/40=1/8

b: =5/6+5/9-1/4

=30/36+20/36-9/36

=41/36

c: =9/10-2/5+3/10+7/20

=12/10-2/5+7/20

=7/20+6/5-2/5

=7/20+4/5

=7/20+16/20

=23/20

\(\dfrac{1}{20}+\dfrac{3}{5}=\dfrac{1}{20}+\dfrac{12}{20}=\dfrac{13}{20}\\ \dfrac{4}{5}\times\dfrac{20}{9}=\dfrac{4\times4\times5}{5\times9}=\dfrac{4\times4}{9}=\dfrac{16}{9}\\ \dfrac{4}{5}-\dfrac{5}{8}=\dfrac{32}{40}-\dfrac{2}{40}=\dfrac{7}{40}\\ \dfrac{1}{2}:\dfrac{5}{8}=\dfrac{1}{2}\times\dfrac{8}{5}=\dfrac{4\times2}{2\times5}=\dfrac{4}{5}\)

13/20

16/9

7/40

1/80

1/20 + 3/5= 1/20 + 12/20 = 13/20

4/5 x 20/9= 80/45=16/9

4/5:5/8=4/5x8/5=32/10= 16/5

1/2:5/8=1/2x8/5=8/10= 4/5

Bài 1: Tính
a, 1/2 x 2/3 x 3/4 = 1/4
b, 7/8 x 8/9 x 9/10 = 7/10
c, 5/14 x 7/15 x 28/7 = 2/3
d, 2 x 1/2 x 3 x 1/3 x 4 x 1/4 x 5 x 1/5 = 1
Bài 2: Tính
a, 7/20 - ( 5/8 - 3/5 ) = 7/20 - 1/40 = 14/40 - 1/40 = 13/40
b, 5/6 + ( 5/9 - 1/4 ) = 5/6 + 11/36 = 30/36 + 11/36 = 41/36
c, 9/10 - ( 2/5 + 3/10 ) + 7/20 = 9/10 - 7/10 + 7/20 = 2/10 + 7/20 = 4/20 + 7/20 = 11/20.
Bài 3: Tìm x:
a, 1/2 + x = 5/6
x = 5/6 - 1/2
x = 1/3
b, 5/6 - x = 1/3
x = 5/6 - 1/3
x = 1/2
c, x - 1/mấy vậy bạn
d, 1/3 x x = 1/6
x = 1/6 : 1/3
x = 1/2

máy tính dâu sao ko lấy ra

Ta có A:B=1.5.6:1:3:5=2

\(\frac{4^5.9^5-2.6^{10}}{2^{10}.3^9+6^9.20}=\frac{\left(4.9\right)^5-2.2^{10.}.3^{10}}{2^{10}.3^9+2^9.3^9.2^2.5}\)

\(\frac{2^{10}.3^{10}^{ }-2^{11}.3^{10}}{2^{10}.3^9+2^{11}.3^9.5}=\frac{2^{10}.3^{10}\left(1-2\right)}{2^{10}.3^9\left(1+2.5\right)}\)

\(\frac{2^{10}.3^{10}.\left(-1\right)}{2^{10}.3^9.\left(-9\right)}=\frac{-3}{-9}=\frac{1}{3}\)

a) \(\frac{7}{20}-\left(\frac{5}{8}-\frac{2}{5}\right)=\frac{7}{20}-\left(\frac{25}{40}-\frac{16}{40}\right)=\frac{7}{20}-\frac{9}{40}=\frac{14}{40}-\frac{9}{40}=\frac{3}{20}\)

b) \(\frac{5}{6}-\left(\frac{5}{9}-\frac{1}{4}\right)=\frac{5}{6}-\left(\frac{50}{36}-\frac{9}{36}\right)=\frac{5}{6}-\frac{41}{36}=\frac{30}{36}-\frac{41}{36}=-\frac{11}{36}\)

c) \(\frac{9}{10}-\left(\frac{2}{5}+\frac{3}{10}\right)+\frac{7}{20}\)

\(=\frac{9}{10}-\frac{2}{5}+\frac{3}{10}+\frac{7}{20}\)

\(=\frac{18}{20}-\frac{8}{20}+\frac{6}{20}+\frac{7}{20}\)

\(=\frac{23}{20}\)

Mình làm vội nên không chắc nhé ! Nhưng lớp 4 chưa học số ân thật .

a.7/20-(5/8-2/5)                                b.5/6-(5/9-1/4)                                c.9/10-(2/5+3/10)+7/20

=7/20-9/40                                       =5/6-11/36                                     =9/10-7/10+7/20

=5/40=1/8                                        =19/36                                          =2/10+7/20

                                                                                                           =11/20

\(\dfrac{3}{5}.\dfrac{5}{7}+\dfrac{12}{20}.\dfrac{2}{14}+\dfrac{9}{15}.\dfrac{1}{7}\)

\(=\dfrac{3}{7}+\dfrac{3}{35}+\dfrac{3}{35}\)

\(=\dfrac{3}{7}+\dfrac{6}{35}\)

\(=\dfrac{15}{35}+\dfrac{6}{35}\)

\(=\dfrac{21}{35}\)

\(=\dfrac{3}{5}\)