Bài 1 .

3 - 5 + 7 - 9 + ... - 317 + 319 - 321

= 3 + ( -5 ) + 7 + ( -9 ) + ... + ( -317 ) + 319 + ( -321 )

= -2 . 160

= -320

Bài 2 :

a) 5 . | x - 4 | = 25

| x - 4 | = 5

TH1 : x - 4 = 5

x = 9

TH2 : x - 4 = -5

x = -1

Vậy x = 9 hoặc x = -1

E ghi sai đầu bài :

(x²- 5).(x² -10) < 0

\(49,=\dfrac{38}{5}-\left(\dfrac{19}{7}+\dfrac{28}{5}\right)\)

\(=\dfrac{38}{5}-\dfrac{19}{7}-\dfrac{28}{5}\)

\(=\left(\dfrac{38}{5}-\dfrac{28}{5}\right)-\dfrac{19}{7}\)

\(=2-\dfrac{19}{7}=-\dfrac{5}{7}\)

\(50,=\dfrac{25}{81}.\dfrac{15}{22}=\dfrac{125}{594}\)

Câu 50 tus sửa đề 

\(-\dfrac{1}{9}.\dfrac{15}{22}:\dfrac{-25}{9}=-\dfrac{1}{9}.\dfrac{15}{22}.\dfrac{-9}{25}=\dfrac{15}{22}.\dfrac{1}{25}=\dfrac{3}{110}\)

Bài 1 :

Câu 49 . = 38/5 - ( 19/7 + 28/5 )

               = 38/5 - 19/7 - 28/5 

               = ( 38/5 - 28/5 ) - 19/7 

               = 2 - 19/7 = -5/7

Câu 50 . = ( -1/9 : -25/9 ) . 15/22

               = 1/25 . 15/22 = 3/110 

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Câu 9

a) 2/15 + (-3/5)

= 2/15 - 3/5

= 2/15 - 9/15

= -7/15

b) -5/7 + 3/7 . (-2/9)

= -5/7 - 2/21

= -15/21 - 2/21

= -17/21

c) 5/16 - (7/15 - 3/16) + 17/30

= 5/16 - 7/15 + 3/16 + 17/30

= (5/16 + 3/16) + (17/30 - 14/30)

= 1/2 + 1/10

= 5/10 + 1/10

= 6/10

= 3/5

\(a,\dfrac{2}{15}+\left(-\dfrac{3}{5}\right)\\ =\dfrac{2}{15}-\dfrac{3}{5}\\ =\dfrac{2}{15}-\dfrac{9}{15}\\ =-\dfrac{7}{15}\\ b,-\dfrac{5}{7}+\dfrac{3}{7}\times\left(-\dfrac{2}{9}\right)\\ =-\dfrac{5}{7}-\dfrac{2}{21}\\ =-\dfrac{15}{21}-\dfrac{2}{21}\\ =-\dfrac{17}{21}\\ c,\dfrac{5}{16}-\left(\dfrac{7}{15}-\dfrac{3}{16}\right)+\dfrac{17}{30}\\ =\dfrac{5}{16}-\dfrac{7}{15}+\dfrac{3}{16}+\dfrac{17}{30}\\ =\left(\dfrac{5}{16}+\dfrac{3}{16}\right)-\left(\dfrac{17}{15}+\dfrac{17}{30}\right)\\ =\dfrac{1}{2}-\dfrac{17}{10}\\ =\dfrac{5}{10}-\dfrac{17}{10}\\ =-\dfrac{6}{5}.\)

a, \(\frac{2}{5}.\frac{1}{3}-\frac{2}{15}:\frac{1}{5}+\frac{3}{5}.\frac{1}{3}\)

\(=\frac{1}{3}.\left(\frac{2}{5}+\frac{3}{5}\right)-\frac{2}{15}.5\)

\(=\frac{1}{3}.1-\frac{2}{3}\)

\(=\frac{1}{3}-\frac{2}{3}\)

\(=\frac{-1}{3}\)

b, \(\left(6-2\frac{4}{5}\right).3\frac{1}{8}+1\frac{3}{8}:\frac{1}{4}\)

\(=\left(6-\frac{14}{5}\right).\frac{25}{8}+\frac{11}{8}.4\)

\(=\frac{16}{5}.\frac{25}{8}+\frac{11}{2}\)

\(=10+\frac{11}{2}\)

\(=\frac{31}{2}\)

1/3×(3/5+2/5)-2/15×1/5

1/3×1-2/15×1/5

1/3-2/15×1/5

1/3-2/75

25/75-2/75

23/75

(6-14/5)×25/8-11/8:4/1

16/5×25/8-11/8:4/1

10/1-11/8:4/1

10/1-11/8×1/4

10/1-11/32

320/32-11/32

309/32

a: =-5,3*2,1=-11,13

b: =3/4-4/5

=15/20-16/20

=-1/20

a,\(\frac{7}{10}\cdot\frac{4}{9}+\frac{3}{10}\cdot\frac{4}{9}-1\frac{7}{9}\)

\(=\frac{14}{45}+\frac{2}{15}-\frac{16}{9}\)

\(=\frac{14}{45}+\frac{6}{45}-\frac{80}{45}\)

\(=\frac{-60}{45}=\frac{-4}{3}\)

b,\(\frac{-5}{6}+\frac{4}{9}\cdot\left(\frac{5}{4}-\frac{2}{3}\right)\cdot\left(-3\right)^2+\frac{5}{9}\cdot30\%\)

\(=\frac{-5}{6}+\frac{4}{9}\cdot\left(\frac{7}{12}\right)\cdot9+\frac{5}{9}\cdot\frac{3}{10}\)

\(=\frac{-5}{6}+\frac{7}{3}+\frac{1}{6}\)

\(=\frac{-5}{6}+\frac{14}{6}+\frac{1}{6}\)

=\(=\frac{10}{6}=\frac{5}{3}\)

\(10^3+\left[120:\left(2^2.3.5-3^2.5\right)\right]\)

\(=1000+\left[120:\left(4.3.5-9.5\right)\right]\)

\(=1000+120:\left(60-45\right)\)

\(=1000+\left[120:15\right]=1000+8=1008\)

=25.8-60.4

=200-240

=-40

\(5^2.8-60.2^2=25.8-60.4=200-240=\left(-40\right)\)