4.[-2(8:4) + 15(-3) - (-12)]

= 4.[-2.2 - 45 + 12]

= 4.[-4 - 45 + 12]

= 4.[-49 + 12]

= 4.[-37]

= - 148

    3.(25 : 5 - 14 : 2) - 5.(6:2)

= 3.(5 - 7) - 5.3

= 3.(-2) - 15

= - 6 - 15

= -21

   [-15:(-3)] - 3.[2.(5 - 9):3]

= 5 - 3.[(2.-4):3]

= 5 - 3.[-8:3]

= 5 + 24:3

= 5 + 8

= 13

câu cuối là 999,7650928....

rồi mà, câu 3 là 100 

khó quá tui chưa học

Ko bết

\(a,\dfrac{3}{4}-\dfrac{2}{5}+\dfrac{5}{12}=\dfrac{45}{60}-\dfrac{24}{60}+\dfrac{25}{60}=\dfrac{46}{60}=\dfrac{23}{30}\\ b,\dfrac{15}{7}-\dfrac{4}{3}-\dfrac{2}{9}=\dfrac{135}{63}-\dfrac{84}{63}-\dfrac{14}{63}=\dfrac{37}{63}\\ c,\dfrac{3}{5}+\dfrac{2}{5}\times\dfrac{4}{5}=\dfrac{15}{25}+\dfrac{8}{25}=\dfrac{23}{25}\\ d,\dfrac{4}{5}-\left(\dfrac{6}{15}+\dfrac{1}{5}\right)=\dfrac{4}{5}-\dfrac{1}{5}-\dfrac{2}{5}=\dfrac{1}{5}\)

A 23/30

B 37/63

C 23/25

D 22/5

Ta tính nhẩm như sau:

3x4=12     2x5=10    5x3=15     4x2=8

12:3=4      10:2=5    15:3=5      8:2=4

12:4=3      10:5=2     15:5=3     8:4=2

3x4 = 12               2x5 = 10        5x3 = 15                  4x2 =8

12 : 3 = 4              10 : 2 = 5       15 : 3 = 5                8 : 2 = 4

12 : 4 =3               10 :5 = 2        15 : 5 = 3                8 : 4 = 2

`1)1/2:2/3 .... 2/3 : 1/2`

`=>1/2xx3/2 .... 2/3xx2`

`=>3/4 .... 4/3`

Vì `3/4 < 1` và `4/3>1` 

`=>3/4<4/3`

__

`4/7:2/5 ... 4/7 : 3/5`

`=>4/7xx5/2....4/7xx5/3`

`=>20/14...20/21`

`=>10/7...20/21`

Vì `10/7>1` và `20/21<1` 

`=>10/7>20/21`

__

`4/15:4/7....2/5xx10/3`

`=>4/15xx7/4...20/15`

`=>7/15...20/15`

Vì `7<20` nên `7/15<20/15`

__

`5/6...15/18-11/18`

`=>5/6...4/18`

Ta có : MSC : `18`

`5/6 = 15/18`

Vì `15>4` nên `5/6 > 4/18`

`2)2/3=(2xx6)/(3xx6)=12/18`

`7/9=(7xx7)/(9xx7)=49/63`

`6/5=(6xx3)/(5xx3)=18/15`

`2/3=(2xx5)/(3xx5)=10/15`

`5/9=(5xx5)/(9xx5)=25/45`

`49/56=(49:7)/(56:7)=7/8`

`6/8=(6xx7)/(8xx7)=42/56`

`2/9=(2xx7)/(9xx7)=14/63`

`49/56=(49:7)/(56:7)=7/8`

a) 4+2/5=22/5

b) 5/4+2=13/4

c) 3+3/2=9/2

d) 3/4-1/8=5/8

e) 17/15-7/10=13/30

g) 25/12-15/8=5/24

h) 5-4/3=11/3

i)15/4-3=3/4

a)\(\dfrac{4}{1}+\dfrac{2}{5}=\dfrac{20}{5}+\dfrac{2}{5}=\dfrac{22}{5}\)

b)\(\dfrac{5}{4}+\dfrac{2}{1}=\dfrac{5}{4}+\dfrac{8}{4}=\dfrac{13}{4}\)

c)\(\dfrac{3}{1}+\dfrac{3}{2}=\dfrac{6}{2}+\dfrac{3}{2}=\dfrac{9}{2}\)

d)\(\dfrac{3}{4}-\dfrac{1}{8}=\dfrac{6}{8}-\dfrac{1}{8}=\dfrac{5}{8}\)

e)\(\dfrac{17}{15}-\dfrac{7}{10}=\dfrac{34}{30}-\dfrac{21}{30}=\dfrac{13}{30}\)

g)\(\dfrac{25}{12}-\dfrac{15}{8}=\dfrac{50}{24}-\dfrac{45}{24}=\dfrac{5}{24}\)

h)\(\dfrac{5}{1}-\dfrac{4}{3}=\dfrac{15}{3}-\dfrac{4}{3}=\dfrac{11}{3}\)

i)\(\dfrac{15}{4}-\dfrac{3}{1}=\dfrac{15}{4}-\dfrac{12}{4}=\dfrac{3}{4}\)

4/1+2/5= 20/5+2/5= 22/5

5/4+2/1= 5/4+ 8/4= 13/4

3/1+3/2= 6/2+ 3/2= 9/2

6/8- 1/8= 5/8

34/30- 21/30= 13/30

50/24- 45/24= 5/24

5/1- 4/3= 15/3- 4/3= 11/3

15/4- 3/1= 15/4- 12/4= 3/4

a: \(\dfrac{16}{15}\cdot\dfrac{-5}{14}\cdot\dfrac{54}{24}\cdot\dfrac{56}{21}\)

\(=\dfrac{16}{24}\cdot\dfrac{-5}{15}\cdot\dfrac{54}{21}\cdot\dfrac{56}{14}\)

\(=\dfrac{2}{3}\cdot\dfrac{-1}{3}\cdot\dfrac{18}{7}\cdot4\)

\(=\dfrac{-2\cdot2\cdot4}{7}=-\dfrac{16}{7}\)

b: \(\dfrac{7}{3}\cdot\dfrac{-5}{2}\cdot\dfrac{15}{21}\cdot\dfrac{4}{-5}\)

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

\(=\dfrac{5}{3}\cdot\dfrac{4}{2}=\dfrac{5}{3}\cdot2=\dfrac{10}{3}\)

c: \(\left(-\dfrac{2}{5}+\dfrac{1}{3}\right)\left(\dfrac{3}{2}-\dfrac{3}{7}\right)\)

\(=\dfrac{-2\cdot5+3}{15}\cdot\dfrac{3\cdot7-3\cdot2}{14}\)

\(=\dfrac{-7}{15}\cdot\dfrac{15}{14}=\dfrac{-7}{14}=-\dfrac{1}{2}\)

d: \(\left(\dfrac{1}{2}-\dfrac{1}{3}\right)\left(5-\dfrac{1}{4}\right)\)

\(=\dfrac{3-2}{6}\cdot\dfrac{20-1}{4}\)

\(=\dfrac{1}{6}\cdot\dfrac{19}{4}=\dfrac{19}{24}\)

e: \(\left(\dfrac{4}{5}\right)^2=\dfrac{4^2}{5^2}=\dfrac{16}{25}\)