mik cũng đang ko biết bài này nè

mình cũng thế

1/6+1/12+1/20+...+1/110 = 

đây mà toán lớp 1 hả bạn

( 1 + 1 + 1 .... +1 ) . 21 

Có 21 số 1 

Tổng ngoặc là : 21 

Tích là : 21 .21=441  

ấy nehs 

=441

học tốt

...

............

\(A=\frac{1}{1.3}+\frac{1}{3.5}+\frac{1}{5.7}+...+\frac{1}{19.21}+\frac{1}{21.23}\)

\(A=\frac{1}{2}\left(\frac{2}{1.3}+\frac{2}{3.5}+\frac{2}{5.7}+...+\frac{2}{19.21}+\frac{2}{21.23}\right)\)

\(A=\frac{1}{2}\left(1-\frac{1}{3}+\frac{1}{3}-\frac{1}{5}+\frac{1}{5}-\frac{1}{7}+...+\frac{1}{19}-\frac{1}{21}+\frac{1}{21}-\frac{1}{23}\right)\)

\(A=\frac{1}{2}\left(1-\frac{1}{23}\right)\)

\(A=\frac{1}{2}.\frac{22}{23}\)

\(A=\frac{11}{23}\)

16/21 nha

\(A=\left(\frac{1}{3}-1\right)\left(\frac{1}{6}-1\right)\left(\frac{1}{10}-1\right)....\left(\frac{1}{36}-1\right)\)

\(=\frac{-2}{3}.\frac{-5}{6}.\frac{-9}{10}...\frac{-35}{36}\)

\(=-\left(\frac{4}{6}.\frac{10}{12}.\frac{18}{20}...\frac{70}{72}\right)=-\left(\frac{1.4}{2.3}.\frac{2.5}{3.4}.\frac{3.6}{4.5}...\frac{7.10}{8.9}\right)\)

\(=-\left(\frac{1.4.2.5.3.6...7.10}{2.3.3.4.4.5...8.9}\right)=-\frac{\left(1.2.3.4..7\right)\left(4.5.6...10\right)}{\left(2.3.4...8\right)\left(3.4.5...9\right)}=-\frac{1.10}{8.3}=\frac{-10}{24}\)

Đặt biểu thức là A .

Ta có :

\(A=\left(\frac{1}{3}-1\right)\left(\frac{1}{6}-1\right)\left(\frac{1}{10}-1\right)...\left(\frac{1}{36}-1\right)\)

\(A=\frac{-2}{3}\cdot\frac{-5}{6}\cdot\frac{-9}{10}\cdot....\cdot\frac{-36}{36}\)

\(A=-\left(\frac{4}{6}\cdot\frac{10}{12}\cdot\frac{18}{20}\cdot...\cdot\frac{70}{72}\right)\)

\(A=-\left(\frac{1.4}{2.3}\cdot\frac{2.5}{3.4}\cdot\frac{3.6}{4.5}...\frac{7.10}{8.9}\right)\)

\(A=-\left(\frac{1.4.2.5.3.6...7.10}{2.3.3.4.4.5...8.9}\right)\)

\(A=-\frac{\left(1.2.3.4....7\right)\left(4.5.6....10\right)}{\left(2.3.4....8\right)\left(3.4.5....9\right)}\)

\(A=-\frac{1.10}{8.3}\)

\(A=\frac{-10}{24}\)