\(B=\dfrac{1}{2}+\dfrac{1}{6}+\dfrac{1}{12}+...+\dfrac{1}{90}\)

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

\(=\left(1-\dfrac{1}{2}\right)+\left(\dfrac{1}{2}-\dfrac{1}{3}\right)+\left(\dfrac{1}{3}-\dfrac{1}{4}\right)+...+\left(\dfrac{1}{9}-\dfrac{1}{10}\right)\)

\(=\left(1-\dfrac{1}{2}+\dfrac{1}{2}-\dfrac{1}{3}+\dfrac{1}{3}-\dfrac{1}{4}+...+\dfrac{1}{9}-\dfrac{1}{10}\right)\)

\(=\left(1-\dfrac{1}{10}\right)\)

\(=\left(\dfrac{10}{10}-\dfrac{1}{10}\right)\)

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

Chúc bạn học tốt

\(B=\dfrac{1}{2}+\dfrac{1}{6}+\dfrac{1}{12}+...+\dfrac{1}{90}\)

\(B=\dfrac{1}{1\times2}+\dfrac{1}{2\times3}+\dfrac{1}{3\times4}+...+\dfrac{1}{9\times10}\)

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

\(B=\dfrac{1}{1}-\dfrac{1}{10}\)

\(B=\dfrac{9}{10}\)

1/2+1/6+1/12+...+1/90=1/(1.2)+1/(2.3)+1/(3.4)+...+1/(9.10)

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

                                =1-1/10

                                =9/10

9/10 nha bạn

\(\frac{1}{2}+\frac{1}{6}+\frac{1}{12}+..+\frac{1}{90}=\frac{1}{1.2}+\frac{1}{2.3}+\frac{1}{3.4}+...+\frac{1}{9.10}\)

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

dấu "." là nhân nhé

\(A=\frac{1}{2}+\frac{1}{6}+\frac{1}{12}+\frac{1}{20}+...+\frac{1}{90}\)

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

\(A=1-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+\frac{1}{4}-\frac{1}{5}+...+\frac{1}{9}-\frac{1}{10}\)

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

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

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

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

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

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

= 1 - 1/10

= 9/10 

Bạn xem lại chỗ 1/10

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

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

A = 1 - 1/10

A = 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 =))

giúp

=> (1/1.2 + 1/2.3 + 1/3.4 + ... + 1/8.9 + 1/9.10) : x = 9/20

=> (1/1 - 1/2 + 1/2 - 1/3 + 1/3 - 1/4 + ... + 1/8 - 1/9 + 1/9 - 1/10) : x = 9/20

=> (1 - 1/10) : x = 9/20

=> 9/10 : x = 9/20

X = 9/10 : 9/20 = 2

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