Đặt B = 1/3 + 1/9 + 1/27 + 1/81 +1/243 + 1/729 + 1/2187

B x 3 = 3 x ( 1/3 + 1/9 +.......+ 1/729 + 1/2187)

        =  1 + 1/3 + 1/9 +.........+1/243 +1/729 

Lấy B x 3 - B ta có :

B x 3 - B = 1 + 1/3 +1/9+ .........+1/243 + 1/729 - 1/3 + 1/9 +.........+1/729 +1/2187

B x (3 - 1)= 1 - 1/2187

B x  2     =   2186/2187

B       = 2186/2187 : 2 = 1093/2187

\(\left(1+\frac{1}{2}\right)\left(1+\frac{1}{3}\right)\left(1+\frac{1}{4}\right)...\left(1+\frac{1}{99}\right)\)

\(=\frac{3}{2}\times\frac{4}{3}\times...\times\frac{100}{99}\)

\(=\frac{100}{2}=50\)

A = 100/2 = 50

giúp mình với nhé

mk cũng lười lắm ,dài quá,mkcũng k hỉu sory

3/4

3/3=1

10/10=1 tk nhé mk sẽ kb

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

= 2/2 + 2/6 + 2/12+...+2/90 = 2(1/2 +1/6 + 1/12 + ...+ 1/90) = 2(1/1.2 + 1/2.3 + 1/3.4 + ... + 1/9.10) = 2(1 - 1/2 + 1/2 - 1/3 + 1/3 - 1/4 + ... + 1/9 - 1/10) = 2(1 - 1/10) = 2 . 9/10 = 9/5

còn +\(\frac{1}{10}\)

\(\frac{1}{10}\cdot\frac{2}{3}\cdot\frac{3}{4}\cdot\frac{4}{5}\cdot\frac{5}{6}\cdot\frac{6}{5}\cdot\frac{5}{4}\cdot\frac{4}{3}\cdot\frac{3}{2}=\frac{1}{10}\)

\(\frac{1}{10}\times\frac{2}{3}\times\frac{3}{4}\times\frac{4}{5}\times\frac{5}{6}\times\frac{6}{5}\times\frac{5}{4}\times\frac{4}{3}\times\frac{3}{2}\)\(\frac{3}{2}\)

\(=\frac{1}{10}\times\left(\frac{2}{3}\times\frac{3}{2}\right)\times\left(\frac{3}{4}\times\frac{4}{3}\right)\times\left(\frac{4}{5}\times\frac{5}{4}\right)\times\left(\frac{5}{6}\times\frac{6}{5}\right)\)

\(=\frac{1}{10}\times1\times1\times1\times1\)

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

#Chúc bạn học tốt !

#k cho mình nhé ?

\(=\frac{1}{2.3}+\frac{1}{3.4}+....+\frac{1}{11.12}\)

\(=\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+...+\frac{1}{11}-\frac{1}{12}\)

\(=\frac{1}{2}-\frac{1}{12}\)

\(=\frac{5}{12}\)

bn sẽ tinh theo kieeuranhaan 2 nha xin lỗi mik làm bi này rùi nhưng mik quên mik có sacks xem lại

1/2 + 1/4 + 1/8 + 1/16 = 1-1/2 + 1/2 - 1/4 + 1/4 - 1/8 + 1/8 - 1/16 = 1 - 1/16 = 15/16

1/2+1/4+1/8+1/16

=8/16+4/16+2/16+1/16

=15/16

TL: 

Đổi 1 m² = 100 dm² 

Lấy 100 : 5 x 2 = 40 dm² 

Vậy: 2/5 m² = 40 dm² 

k cho tui nhé 

@@@@@@@@@@@@@@@@@ 

HT

TL: 

Đổi: 1 m² = 100 dm² 

Lấy: 100 x \(\frac{2}{5}\)= 40 dm²

Vậy: \(\frac{2}{5}\)m² = 40 dm².

HT