Giải hộ mik nha mấy bạn

\(\frac{1}{6}+\frac{1}{12}+\frac{1}{20}+...+\frac{1}{90}\)

\(\frac{1}{6}+\frac{1}{12}+\frac{1}{30}+\frac{1}{42}+...+\frac{1}{90}\)

\(=\frac{1}{1.2}+\frac{1}{2.3}+\frac{1}{3.4}+\frac{1}{4.5}+...+\frac{1}{9.10}\)

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

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

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

 S= 1/2 + 1/6 + 1/12 + ...+ 1/90

S=1/1.2 + 1/2.3 +1/3.4+...+1/9.10

S=1-1/2 + 1/2-1/3 + 1/3-1/4+...+1/9-1/10

S=(1-1/10)+(1/2-1/2)+(1/3-1/3)+...+(1/9-1/9)

S=9/10+0+0+...+0

S=9/10Chúc bn học tốt =))

\(\frac{1}{2}+\frac{5}{6}+\frac{11}{12}+...+\frac{89}{90}\)

= \(\left(1-\frac{1}{2}\right)+\left(1-\frac{1}{6}\right)+\left(1-\frac{1}{12}\right)+...+\left(1-\frac{1}{90}\right)\)

= \(\left(1+1+...+1\right)-\left(\frac{1}{2}+\frac{1}{6}+\frac{1}{12}+...+\frac{1}{90}\right)\)8 số hạng 1

= \(\left(1.8\right)-\left(\frac{1}{1.2}+\frac{1}{2.3}+...+\frac{1}{9.10}\right)\)

= \(8-\left(1-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+...+\frac{1}{9}-\frac{1}{10}\right)\)

= \(8-\left(1-\frac{1}{10}\right)\)

= \(8-\frac{9}{10}\)

= \(\frac{71}{10}\)

\(S=\dfrac{1}{6}+\dfrac{1}{12}+...+\dfrac{1}{600}+\dfrac{1}{650}\)

\(=\dfrac{1}{2\cdot3}+\dfrac{1}{3\cdot4}+...+\dfrac{1}{24\cdot25}+\dfrac{1}{25\cdot26}\)

\(=\dfrac{1}{2}-\dfrac{1}{3}+\dfrac{1}{3}-\dfrac{1}{4}+...+\dfrac{1}{24}-\dfrac{1}{25}+\dfrac{1}{25}-\dfrac{1}{26}\)

\(=\dfrac{1}{2}-\dfrac{1}{26}=\dfrac{13-1}{26}=\dfrac{12}{26}=\dfrac{6}{13}\)

=1/1x2+1/2x3+1/3x4+.....+1/9x10

=1/1-1/2+1/2-1/3+1/3-1/4+.........-1/10

=1/1-1/10

=1/9

1/2+1/6+....+1/90

=1/1.2 +1/2.3 +1/3.4+.....+1/9.10

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

=1-1/10=9/10

k 3 cái sao đc

\(A=\frac{1}{2.3}+\frac{1}{3.4}+\frac{1}{4.5}+...+\frac{1}{8.9}=\frac{1}{9.10}\)

tới đây chắc bạn giải đc rồi ha!

nếu k biết cứ nhắn hỏi mình

bn giải giùm mình ghi các bước ra nha

bn giải hết giùm mình

S= \(\frac{1}{2.3}+\frac{1}{3.4}+\frac{1}{4.5}+\frac{1}{5.6}+\frac{1}{6.7}+\frac{1}{7.8}\)

S= 1/2 - 1/3 + 1/3 - 1/4 + 1/4 - 1/5 +  1/5 -1/6 +1/6 - 1/7 + 1/7 - 1/8

S= 1/2 - 1/ 8 

S= 3/8

S= 1/2.3 + 1/3.4 + 1/4.5 + 1/5.6 + 1/6.7 + 1/7.8

  = 1/2 - 1/3 + 1/3 - ...+ 1/7 - 1/8

  = 1/2 - 1/8

  = 3/8

S=(1/2.3) +(1/3.4) + (1/4.5) +...+1/7.8

S=1/2-1/3+1/3-1/4 +1/4-1/5+...+1/7-1/8

S=1/2-1/8

S=3/8