30 = 6 . 5

60 = 6 . 10

36 = 6 . 6

6⁴ . 5² . 2 = 1296 . 50 = 64800

BCNN(30;60;36)=64800

a, \(\frac{1}{2}+\frac{1}{4}+\frac{1}{8}+\frac{1}{16}+...+\frac{1}{512}\)

Đặt A = \(\frac{1}{2}+\frac{1}{4}+\frac{1}{8}+\frac{1}{16}+...+\frac{1}{512}\)

\(A\cdot2=1+\frac{1}{2}+\frac{1}{4}+\frac{1}{8}+...+\frac{1}{256}\)

\(A\cdot2-A=\left(1+\frac{1}{2}+\frac{1}{4}+\frac{1}{8}+\frac{1}{16}+..+\frac{1}{256}\right)-\) \(\left(\frac{1}{2}+\frac{1}{4}+\frac{1}{8}+\frac{1}{16}+...+\frac{1}{512}\right)\)

\(A=1+\frac{1}{2}+\frac{1}{4}+\frac{1}{8}+...+\frac{1}{256}-\frac{1}{2}-\frac{1}{4}-\frac{1}{8}-\frac{1}{16}-...-\frac{1}{512}\)

\(A=1-\frac{1}{512}\)

\(A=\frac{511}{512}\)

b, \(\frac{1}{1\cdot2}+\frac{1}{2\cdot3}+\frac{1}{3\cdot4}+...+\frac{1}{9\cdot10}\) ( mình  ... vì lưới quá không muốn đánh nhiều )

\(=\frac{1}{1}-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+...+\frac{1}{9}-\frac{1}{10}\)

\(=\frac{1}{1}-\frac{1}{10}\)

\(=\frac{9}{10}\)

\(\left(1-\frac{1}{2}\right)\left(1-\frac{1}{3}\right)\left(1-\frac{1}{4}\right).....\left(1-\frac{1}{10}\right)\)

\(=\frac{1}{2}\cdot\frac{2}{3}\cdot\frac{3}{4}\cdot....\cdot\frac{9}{10}\)

\(=\frac{1\cdot2\cdot3\cdot....\cdot9}{2\cdot3\cdot4\cdot....\cdot10}\)

\(=\frac{1}{10}\)

\(5,3+6,9+5,8=18\)

6 : 6 + 6 : 6

Nhân chia trước, cộng trừ sau nhé bạn.

Học tốt, nhớ cho mình đúng nha.

6 / 6 + 6 / 6 = 2

4/6

5/6

1/2<x/6<1

=> 3/6<x/6<6/6

=> x/6=4/6;5/6

ta có 

\(1=\frac{899}{900}+\frac{1}{900}\)

\(1=\frac{1235}{1236}+\frac{1}{1236}\)

vì \(\frac{1}{900}\) > \(\frac{1}{1236}\) ( phần bù nha )

nên => \(\frac{899}{900}< \frac{1235}{1236}\)

\(\frac{899}{900}=\frac{900-1}{900}=\frac{900}{900}-\frac{1}{900}=1-\frac{1}{900}\)    

\(\frac{1235}{1236}=\frac{1236-1}{1236}=\frac{1236}{1236}-\frac{1}{1236}=1-\frac{1}{1236}\)   

\(\frac{1}{900}>\frac{1}{1236}\)   

\(1-\frac{1}{900}< 1-\frac{1}{1236}\)   

\(\frac{899}{900}< \frac{1235}{1236}\)

1,08km=108000m

1,08km​​\(=\)1080m