1+2+3+4+5+6+7+8+9 = ( 1+9) + ( 2+8) + (3+7) + (4+6) +5

= 10 + 10 + 10 + 10 + 5

= 45

(1+9)+(2+8)+(3+7)+(4+6)+5
=10 + 10 + 10 + 10
=40

\(=\left(\dfrac{1}{6}+\dfrac{5}{6}\right)+\left(\dfrac{4}{9}+\dfrac{5}{9}\right)+\left(\dfrac{11}{16}+\dfrac{5}{16}\right)=1+1+1=3\)

\(=\left(\dfrac{1}{6}+\dfrac{5}{6}\right)+\left(\dfrac{4}{9}+\dfrac{5}{9}\right)+\left(\dfrac{11}{16}+\dfrac{5}{16}\right)\\ =1+1+1=3\)

= (1/6+5/6)+(4/9+5/9)+(11/16+5/16)

= 1+1+1

=3

\(=\dfrac{1}{2}\cdot\dfrac{2}{3}\cdot\dfrac{6}{5}\cdot\dfrac{5}{6}=\dfrac{1}{2}\cdot\dfrac{2}{3}=\dfrac{1}{3}\)

\(\dfrac{1}{2}\times\dfrac{2}{3}:\dfrac{5}{6}\times\dfrac{5}{6}\)

\(=\dfrac{1}{2}\times\left(\dfrac{2}{3}:\dfrac{5}{6}\times\dfrac{5}{6}\right)\)

\(=\dfrac{1}{2}\times\dfrac{2}{3}\)

\(=\dfrac{1}{3}\)

bài2:Tìm y:

a)y+253=1420                                 b)y-2316=902
y=1420-253                                        y=902+2316
y=1167                                               y=3218

bài3:Tính:

2754:9=306                    5089:7=727                   3675:5=735                   2010:6=335

2754x9=24786               5089x7=35623              3675x5=18375             2010x 6=12060

[1+19]+[4+16]+[7+13]+[22+28]+10+25=

20+20+20+50+35=145

y+253=1420

y        =1420-253

y        =1267

y-2316=902

y         =902+2316

y           =3218

1/2 + 1/3 + 1/4 + 1/5 + 4/5 + 3/4 + 2/3 + 1/2

= (1/2 + 1/2) + (1/3 + 2/3) + (1/4 + 3/4) + (1/5 + 4/5)

= 1 + 1 + 1 + 1

= 4

kết quả = 9/2

TL

1+2+3+4+5+6+7+8+9=45

nha bn

HT

1+2+3+...+9

=(1+9)+(2+8)+...+5

=10+10+...+5

=10.4+5

=45

HT

45

nha bn

HT

OK

+1

A = 1 + 2 +  2x2 + 2x2x2 + 2x2x2x2 + ... + 2x2x...x2 _{^{9 lần số 2}}

=> 2xA = 2 + 2x2 + 2x2x2 + ... + 2x2x...x2 _{^{( 10 lần số 2)}}

=> 2xA - A = 2 + 2x2 + 2x2x2 + ... + 2x2x...x2 _{^{10 lần số 2}} - ( 1 + 2 +  2x2 + 2x2x2 + 2x2x2x2 + ... + 2x2x...x2 _{^{9 lần số 2}} )

=> A = 2x2x2x...x2 - 1_{^{10 lần số 2}} = 512x2 - 1 = 1024 - 1 = 1023

(9+1) + ( 2 + 8 ) + ( 3 + 7) + ( 4 + 6 ) + 5
=10 + 10 + 10 + 10 + 5 
= 10 x 4 + 5
= 40 + 5
= 45

1+2+3+4+5+6+7+8+9

= ( 1 + 9 ) +  ( 2 + 8 ) + ( 3 + 7 ) + ( 4 + 6 ) + 5

= 10  x 4 + 5

= 45

~HT~

@Anh

1 + 2 + 3 + 4 + 5 + 6 + 7 +8 +9

=(1+9) + (2+8) + (3+7) + (4+6) + 5

=10 + 10 + 10 + 10 + 5

=45

Nhớ k đúng cho mk nha, thanks

--------_HT_---------