28 phút 15 giây

54 phút 39 giây - 26 phút 24 giây = 28 phút 15 giây

28 phút 15 giây nha

1) -39 +14+26+39

=   39-39 + 40

=40

2)  -54 +16 +(-6)+54

=54-54  +10

=10

3) 45 +23+(-35)+(-23)

=  45-35 +23 -23

=10+0

=10

4) 45  .17 - 5.9.7

=45.17 -45.7

=45.( 17-7)

=45 .10

=450

5) 24 ( 16-5) -16(24-5)

=24.16 -24.5 -16 .24 +16 .5

= 24.16 -16.24 +5 (16-24)

=0  +5 .(-8)

=-40

Chúc hok tốt!

1. ( - 39 ) + 14 + 26 + 39 

= [ ( - 39 ) + 39 ] + ( 14 + 26 )
= 0 + 40 

= 40

2. ( - 54 ) + 16 + ( - 6 ) + 54

= [ ( - 54 ) + 54 ] + [ 16 + ( - 6 ) ]

= 0 + 10

= 10

3. 45 + 23 + ( - 35 ) + ( - 23 )

= [ 45 + ( - 35 ) ] + [ 23 + ( - 23 ) ] 

= 10 + 0 

= 10

4. 45 . 17 - 5 . 9 . 7

= 45 . 17 - 45 . 7 

= 45 . ( 17 - 7 )
= 45 . 10

= 450

5. 24 . ( 16 - 5 ) - 16 . ( 24 - 5 )

= 24 . 11 - 16. 19

= 264 - 304

= 264 + ( - 304 )

= - ( 304 - 364 ) 

= - 40

Chúc bạn học tốt ! 

1. (-39)+14+26+39

\(=\left[\left(-39\right)+39\right]+\left(14+26\right)\)

\(=0+40\)

\(=40\)

2. (-54)+16+(-6)+54

\(=\left[\left(-54\right)+54+\left[16+\left(-6\right)\right]\right]\)

\(=0+10\)

\(=10\)

3. 45+23+(-35)+(-23)

\(=\left(45+\left(-35\right)\right)+\left(23+\left(-23\right)\right)\)

\(=10+0\)

\(=10\)

4. 45.17-5.9.7

\(=45.17-45.7\)

\(=45.\left(17-7\right)\)

\(=45.10\)

\(=450\)

5. 24.(16-5)-16.(24-5)

\(=24.16-24.5-16.24+16.5\)

\(=24.\left(16-16\right)+5.\left(-24+16\right)\)

\(=24.0+5.\left(-8\right)\)

\(=0+\left(-40\right)\)

\(=-40\)

<1/25+1/25+...+1/25+1/40+1/40+1/40+...+1/40

=1/25.15+1/40.15=15/25+15/40=3/5+3/8=39/40

đặt vế trái là A ta có A<1/25.16+1/45.14=214/225=1-11/225 39/40=1-1/40 ta có 1.225<11.40=>1/40<11/225=>1-1/40>1-11/225=>214/225<39/40 mà A<214/225=>A<39/40(đpcm nhớ k cho mk nha

=1/25+1/54

=79/1350

vi 79/1350<39/40

nen 1/25+1/26+1/27+...1/54<39/40

đặt vế trái là A ta có

A<1/25.16+1/45.14=214/225=1-11/225

39/40=1-1/40

ta có 1.225<11.40=>1/40<11/225=>1-1/40>1-11/225=>214/225<39/40 mà A<214/225=>A<39/40(đpcm

nhớ k cho mk nha

Ta có : 1/25 + 1/26 + 1/27 +.....+1/39 < 1/25 + 1/25 + .....+1/25 = 15/25 = 3/5

1/40+1/41 +.....+1/54 < 1/40 + 1/40 +....+1/40 = 15/40 = 3/8

=> A = 1/25 + 1/26 + 1/27 +.......+1/54 < 3/5 + 3/8 = 39/40

=> A < 39/40   (đpcm)

3. 31-[26-(2017+35)]

=31-26-2017+35

=(31-26+35)-2017

=40-2017

=-1977

4. 54+(-37+10-54+67)

=54-37+10-54+67

=(54-54)-(37-67)+10

=0+30+10

=40

 481 : (21 + 16) x 23 

= 481 : 37 x 23 

= 13 x 23 

= 299

39 150 : 54 + 9906 : 26

= 725 + 381

=1 106

B

`=>B`

b