Ta có

1/2 + 2/6 + 3/9 + 4/12 + 5/15 + 6/18 

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

= 1/2 + 5/3

= 3/6 + 10/6 

= 13/6 

t nha bn

1/2+2/6+3/9+4/12+5/15+6/18=13/6

Ta có: 1+2+3+…+20=20.(20+1):2=20.21:2=420:2=210

khoảng cách: 2-1=1

số số hạng là: (20-1):1+1=20(số)

tổng là: (20+1)x20:2=210

a 7/15 + (5/6 - 1/3) = 7/15 + 1/2 = 14/30 + 15/30 = 29/30

b 4/7 *21/16 : 5/8 = 3/4  : 5/8 = 6/5

c 15/8 - 4/9 * 6/8+ 1/6 =15/8 - 1/3 + 1/6 = 37/24 + 1/6 = 41/24

d 5/12+(3/4 + 2/3 ) : 34/3 =

a/\(\frac{7}{15}+\left(\frac{5}{6}-\frac{1}{3}\right)=\frac{7}{15}+\frac{1}{2}=\frac{29}{30}\)

b/\(\frac{4}{7}\cdot\frac{21}{16}:\frac{5}{8}=\frac{4}{7}\cdot\frac{2}{16}\cdot\frac{8}{5}=\frac{4}{35}\)

c/\(\frac{15}{8}-\frac{4}{9}\cdot\frac{6}{8}+\frac{1}{6}=\frac{15}{8}-\frac{1}{3}+\frac{1}{6}=\frac{41}{24}\)

d/ \(\frac{5}{12}+\left(\frac{3}{4}+\frac{2}{3}\right):\frac{34}{3}=\frac{5}{12}+\frac{17}{12}:\frac{34}{3}=\frac{5}{12}+\frac{1}{8}=\frac{13}{24}\)

Đặt A= 1+ 2+ 3+...+ 19+ 20.

A có số các số hạng là:

( 20- 1): 1+ 1= 20( số)

A là:

( 20+ 1)x 20: 2= 210

Vậy A= 210.

1+2+3+4+5+6+7+8+9+10+11+12+13+14+15+16+17+18+19+20=210

kho qua minh khong biet lam

a) 1/2 = 2/4 = 3/6 = 4/8 = 5/10

b) 1/3 = 2/6 = 3/9 = 4/12 = 5/15

c) 3/4 = 6/8 = 9/12 = 12/16 = 18/24

d) 4/3 = 8/6 = 12/9 = 16/12 = 20/15

e) 6/4 = 3/2 ; 6/3 = 4/2

g) 4/2 12/6 = 24/12