1/2+1/6+1/12+1/20+...+1/380

=1/1.2+ 1/2.3+ 1/3.4+ 1/4.5+...+ 1/19.20

= 1/1-1/2+ 1/2-1/3+ 1/3-1/4+ 1/4-1/5+...+1/19-1/20

=1-1/29

=29/29-1/29

= 28/29

28/29

\(\frac{1}{2}+\frac{1}{6}+\frac{1}{12}+\frac{1}{20}+...+\frac{1}{380}\)

\(=\frac{1}{1\times2}+\frac{1}{2\times3}+\frac{1}{3\times4}+\frac{1}{4\times5}+\frac{1}{19\times20}\)

\(=\frac{1}{1}-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+\frac{1}{4}-\frac{1}{5}+...+\frac{1}{19}-\frac{1}{20}\)

\(=1-\frac{1}{29}\)

\(=\frac{28}{29}\)

AI GIÚP MÌNH VỚI MÌNH ĐANG CẦN GẤP

1/3+1/6+1/12+1/24=8/24+4/24+2/24+1/24=15/24=5/8

tk cho mk nha!

\(\frac{1}{3}+\frac{1}{6}+\frac{1}{12}+\frac{1}{24}=\frac{8}{24}+\frac{4}{24}+\frac{2}{24}+\frac{1}{24}=\frac{15}{24}=\frac{5}{8}\)

\(\frac{5}{8}\)

~Hok tốt~

nhớ ghi rõ cách làm ( tính nhanh )

Ta có : \(\frac{3}{5}+\frac{6}{11}+\frac{7}{13}+\frac{2}{5}+\frac{5}{11}+\frac{19}{13}+\frac{1}{2}+\frac{2}{3}-\frac{1}{6}\)

\(=\left(\frac{3}{5}+\frac{2}{5}\right)+\left(\frac{6}{11}+\frac{5}{11}\right)+\left(\frac{7}{13}+\frac{19}{13}\right)+\left(\frac{1}{2}+\frac{2}{3}-\frac{1}{6}\right)\)

\(=1+1+2+1=5\)

\(\frac{3}{5}+\frac{6}{11}+\frac{7}{13}+\frac{2}{5}+\frac{5}{11}+\frac{19}{13}:\frac{1}{2}+\frac{2}{3}-\frac{1}{6}\)

\(=\left(\frac{3}{5}+\frac{2}{5}\right)+\left(\frac{19}{13}-\frac{7}{13}\right)+\left(\frac{6}{11}+\frac{5}{11}\right):\frac{2}{3}+\frac{1}{6}\)

\(=1+1+1:\frac{2}{3}+\frac{1}{6}\)

\(=3:\frac{2}{3}+\frac{1}{6}\)

\(=\frac{9}{2}+\frac{1}{6}\)

\(=\frac{14}{3}\)

= (3/5 + 2/5) + (6/11 + 5/11) + (7/13 + 19/13) : 1/2 + 2/3 - 1/6

= (1 + 1 + 2) : (1/2 + 4/6 - 1/6)

= 4 : (1/2 + 1/2)

= 4 : 1

= 4

Vậy kết quả bằng 4

giúp mk với

mong ban ghi han hoi :)

khó đọc quá

Đặt A=1+1/3+1/6+1/10+ ... + 1/45

1/2.A=1/2+1/6+1/12+1/20+ ... + 1/90

1/2.A= 1/1.2+1/2.3+1/3.4+1/4.5+ ... + 1/9.10

1/2A= 1-1/2+1/2-1/3+1/3-1/4+1/4-1/5+ ... +1/9-1/10

1/2.A= 1-1/10

1/2.A=9/10

A=(9/10).2

A=9/5

Sai thì thôi đấy 

bài này lớp 4 mà mình không làm được nhỉ . mình học dốt dốt quá .