1+2+3+4+5+6+7+8+9+10+11+12+13+14+15+16+17+18+19+20

=(1+19)+(2+18)+(3+17)+(4+16)+(5+15)+(6+14)+(7+13)+(8+12)+(9+11)+20+10

=20+20+20+20+20+20+20+20+20+20+10

=20×10+10

=200+10

=210

= ( 1 + 19 ) + ( 2 + 18 ) + ( 3 + 17 ) + (4 + 16 ) + ( 5 + 15 ) + ( 6 + 14 ) + ( 7 + 13 ) + ( 8 + 12) + ( 9+ 11 ) + 20

= 20 + 20 + 20 +20 +  20 +20 + 20 + 20 + 20 +20

= 20 x 10

= 200

tui đầu tiên đó 

Ta có : \(1+2+3+...+19+20\)

          \(=\frac{\left(20+1\right).20}{2}=\frac{21.20}{2}=210\)

(19+1)+(18+2)+(17+3)+(16+4)+(15+5)+(14+6)+(13+7)+(12+8)+(11+9)+20)+10

=20+20+20+20+20+20+20+20+20+20=(20*10)+10

=210

1+2+3+4+5+6+7+8+9+10+11+12+13+14+15+16+17+18+19+20

=(1+19)+(2+18)+(3+17)+(4+16)+(5+15)+(6+14)+(7+13)+(8+12)+(9+11)+(10+20)

=20+20+20+20+20+20+20+20+20+30

=(20 x 9) + 30

=180 + 30

=210

tk nha

1+2+3+4+5+6+7+8+9+10+11+12+13+14+15+16+17+18+19+20

= (1+19)+(2+18)+(3+17)+(4+16)+(5+15)+(6+14)+(7+13)+(8+12)+(9+11)+20+10

= 20     +   20   +  20     +  20    +  20      +  20    +  20     +  20    +  20     +  20 + 10

= 10 . 10 +10 

=  100     +  10 = 110

a: \(=\dfrac{21}{11}+\dfrac{12}{11}+\left(-\dfrac{9}{19}-\dfrac{3}{19}-\dfrac{6}{19}\right)=3-\dfrac{18}{19}=\dfrac{39}{19}\)

b: \(=\dfrac{2}{3}+\dfrac{4}{3}+\dfrac{3}{4}+\dfrac{7}{4}+\dfrac{4}{5}+\dfrac{16}{5}=2+4+\dfrac{5}{2}=6+\dfrac{5}{2}=\dfrac{17}{2}\)

TL: 

8/15 x 5/6 = 40/90 = 4/9 

41/20 x 3/4 = 123/80 

(11/23 + 9/23) + (18/23 + 2/23) = 20/23 + 20/23 = 40/43 

HT

\(\left(\frac{2}{3}\times\frac{4}{5}\right)\times\frac{5}{6}\) = \(\frac{2}{3}\times\frac{4}{5}\times\frac{5}{6}\)=\(\frac{2\times4\times5}{3\times5\times6}\)=\(\frac{2\times4\times5}{3\times5\times2\times3}\)=\(\frac{4}{9}\)

\(\frac{11}{23}+\frac{2}{23}+\frac{9}{23}+\frac{18}{23}\)=\(\frac{\left(11+9\right)+\left(18+2\right)}{23}\)=\(\frac{20+20}{23}\)=\(\frac{40}{23}\)

\(\frac{27}{12}+\frac{17}{6}-\frac{25}{36}-\frac{15}{6}\)=\(\left(\frac{27}{12}-\frac{25}{36}\right)+\left(\frac{17}{6}-\frac{15}{6}\right)\)=\(\frac{14}{9}+\frac{2}{6}\)=\(\frac{17}{9}\)

/HT\

Nhập các bạn toàn bắt tớ làm à ngu gò mà làm

a.245 x ( 327-18-9) = 245 x 300 = 73500

b. (462 - 62) x 4 = 1600

c. 900 : 9 = 100

d. (12+20) : 4 = 8

462 x 4 - 62 x 4

= 4 x ( 462 - 62 )

= 4 x 400

= 1600

35 : 7 - 21 :7

= ( 35 - 21 ) : 7

=  14 : 7

= 2

( 25 x 36 ) : 9

= 25 x ( 36 : 9 )

= 25 x 4

= 100

245 x 327 - 245 x 18 - 9 x 245

= 245 x ( 327 - 18 - 9 )

= 245 x 218

= 53410

12 : 4 + 20 : 4

= ( 12 + 20 ) : 4

= 24 : 4

Chúc em học tốt \ ( ^ o ^ ) / tick cho nha

245x327-245x18-9x245                           462 x 4 - 62 x 4   

=245x(327-18-9)                                       = ( 462 - 62) x 4

=245x300                                                  = 400 x 4

=73500                                                      = 1600

   35 : 7 - 21 : 7                                             ( 25 x 36 ) : 9    

= ( 35 - 21) : 7                                           = 900 : 9

= 14 : 7                                                     = 100

= 2

12 : 4 + 20 : 4

= ( 12 + 20 ) : 4

= 32 : 4

= 8

\(11-12+13-14+15-16+17-18+19-20+21+22\)

\(=\left(11+13+15+17+19+21+22\right)-\left(12+14+16+18+20\right)\)

\(=\left(A+22\right)-B\)

     số số hạng của tổng A là :

         ( 21 - 11 ) :2 + 1 = 6 ( số )

    vậy tổng A là :

     ( 21 + 11 ) * 6 :2 = 96 

  số số hạng của tổng B là :

     (20-12 ) :2 +1 = 5 ( số )

tổng B là :

     ( 20 + 12 ) * 5 :2 = 80 

   vậy \(\left(A+22\right)-B\)

   \(=\left(96+22\right)-80\)

\(=118-80\)

\(=38\)

\(11-12+13-14+15-16+17-18+19-20+21+22\)

\(=\left(11-12\right)+\left(13-14\right)+\left(15-16\right)+\left(17-18\right)+\left(19-20\right)+21+22\)

\(=-1+-1+-1+-1+-1+21+22\)

\(=-5+21+22\)

\(=38.\)

21-20+19-18+17-16+15-14+13-12+11+22=1+1+1+1+1+33=38

Trả lời

Sửa đề câu a 1 tí

a)39/16:5/8-17/16:5/8+4/5

=(39/16-17/16):5/8+4/5

=11/8:5/8+4/5

=11/5+4/5

=15/5=3

b)16/17.12/19+16/17.7/19-2/3

=16/17.(12/19+7/19)-2/3

=16/17-2/3

=48/56-14/56

=34/56=17/28

Học tốt !

`(2/3xx4/5)xx5/6`

`=2/3xx4/5xx5/6`

`=2/3xx4/6`

`=2/3xx2/3`

`=1`

__

`(4/5+5/4)xx3/4`

`=(16/20+25/20)xx3/4`

`=41/20xx3/4`

`=123/80`

__

`27/12+17/6-25/36-15/6`

`=(27/12-25/36)+(17/6-15/6)`

`=(81/36-25/36)+2/6`

`=56/36+2/6`

`=56/36+12/36`

`=68/36`

`=17/9`

\(1/\)

\(\frac{12}{49}:\frac{16}{21}\)

\(=\frac{12}{49}\times\frac{21}{16}\)

\(=\frac{12\times21}{49\times16}\)

\(=\frac{252}{784}=\frac{9}{28}\)

\(2/\)

\(a,\frac{5}{11}-\left(\frac{39}{61}+\frac{12}{61}\right)\)

\(=\frac{5}{11}-\frac{51}{61}\)

\(=-\frac{256}{671}\)

\(b,\frac{17}{12}+\frac{9}{7}-\left(\frac{2}{7}-\frac{7}{12}\right)\)

\(=\frac{17}{12}+\frac{9}{7}-\frac{2}{7}+\frac{7}{12}\)

\(=\left(\frac{17}{12}+\frac{7}{12}\right)+\left(\frac{9}{7}-\frac{2}{7}\right)\)

\(=2+1\)

\(=3\)

\(c,\frac{16}{17}\times\frac{12}{19}+\frac{16}{17}\times\frac{7}{19}\)

\(=\frac{16}{17}\times\left(\frac{12}{19}+\frac{7}{19}\right)\)

\(=\frac{16}{17}\times1\)

\(=\frac{16}{17}\)

\(3/\)

\(a,x:\frac{4}{5}=\frac{25}{8}:\frac{5}{4}\)

\(x:\frac{4}{5}=\frac{5}{2}\)

\(x=2\)

\(b,\frac{22}{5}:x=\frac{44}{5}:\frac{5}{2}\)

\(\frac{22}{5}:x=\frac{88}{25}\)

\(x=\frac{5}{4}\)

\(c,x\times\frac{4}{9}\times\frac{15}{18}=\frac{25}{18}\)

\(x\times\frac{4}{9}=\frac{5}{3}\)

\(x=\frac{15}{4}\)

Còn bài 4 bạn tham khảo tại link này nhé:

Câu hỏi của Phạm Huy Bảo - Toán lớp 5 - Học toán với OnlineMath