\(A=\dfrac{1}{3.4}+\dfrac{1}{4.5}+\dfrac{1}{5.6}+\dfrac{1}{6.7}+\dfrac{1}{7.8}\)

\(A=\dfrac{1}{3}-\dfrac{1}{4}+\dfrac{1}{4}-\dfrac{1}{5}+...+\dfrac{1}{7}-\dfrac{1}{8}\)

\(A=\dfrac{1}{3}-\dfrac{1}{8}\)

\(A=\dfrac{8}{24}-\dfrac{3}{24}\)

\(A=\dfrac{5}{24}\)

A=1/3.4+1/4.5+1/5.6+1/6.7+1/7.8

=\(\dfrac{1}{3}-\dfrac{1}{4}+\dfrac{1}{4}-\dfrac{1}{5}+\dfrac{1}{6}-\dfrac{1}{6}+\dfrac{1}{7}-\dfrac{1}{8}\)

=\(\dfrac{1}{3}-\dfrac{1}{8}\)=\(\dfrac{8-3}{24}\)=\(\dfrac{5}{24}\)

A=1/3.4+1/4.5+1/5.6+1/6.7+1/7.8

A=1/3-1/4+1/4-1/5+1/5-1/6+1/6-1/7+1/7-1/8

A=1/3-(1/4-1/4+1/5-1/5+1/6-1/6+1/7-1/7)-1/8{mk ko bít viết phân số thông cảm nha}

A=1/3-0-1/8

A=1/3-1/8

A=5/24

6,45

\(\dfrac{1}{2.3}+\dfrac{1}{3.4}+\dfrac{1}{4.5}+\dfrac{1}{5.6}+\dfrac{1}{6.7}\\ =\dfrac{1}{2}-\dfrac{1}{3}+\dfrac{1}{3}-\dfrac{1}{4}+\dfrac{1}{4}-\dfrac{1}{5}+\dfrac{1}{5}-\dfrac{1}{6}+\dfrac{1}{6}-\dfrac{1}{7}\\ =\dfrac{1}{2}-\dfrac{1}{7}\\ =\dfrac{5}{14}\)

ko spam

lèm rùi mè

=1/3 - 1/4 +1/4-1/5 +...+1/98 - 1/99

=1/3-1/99=32/99

A=1/3-1/4+1/4-1/5+....+1/95-1/96

=1/3-1/96=31/96

1/3-1/4+1/4-1/5+1/5-1/6+......+1/95-1/96

1/3-1/96

32-1/96

31/96

B=1/3-1/4+1/4-1/5+...+1/95-1/96

B=1/3-1/96

B=31/96

Dễ thôi bạn!

 1/3.4+1/4.5+1/5.6+...+1/99.100

=1/3-1/4+1/4-1/5+1/5-1/6+...+1/98-1/99+1/99-1/100

=1/3-1/100

=97/300

\(\frac{1}{3.4}+\frac{1}{4.5}+\frac{1}{5.6}+...+\frac{1}{99.100}\)

\(=\frac{1}{3}-\frac{1}{4}+\frac{1}{4}-\frac{1}{5}+\frac{1}{5}-\frac{1}{6}+...+\frac{1}{99}-\frac{1}{100}\)

\(=\frac{1}{3}-\frac{1}{100}\)

\(=\frac{97}{300}\)

1/3.4 + 1/ 4.5 + 1/5.6 + ... + 1/99.100

= 1/3 - 1/4 + 1/4 - 1/5 + 1/5 - 1/6 + ... + 1/99 - 1/100

= 1/3 -1/100

= 97/300

=1-1/2+1/2-1/3+......+1/5-1/6

=1-1/6

=5/6

Tick

\(\dfrac{1}{3.4}+\dfrac{1}{4.5}+\dfrac{1}{5.6}+...+\dfrac{1}{n.\left(n+1\right)}=\dfrac{3}{10}\)

Ta có: \(\dfrac{1}{3}-\dfrac{1}{4}+\dfrac{1}{4}-\dfrac{1}{5}+\dfrac{1}{5}-\dfrac{1}{6}+...+\dfrac{1}{n}-\dfrac{1}{n+1}=\dfrac{3}{10}\)

\(\dfrac{1}{3}-\dfrac{1}{x+1}=\dfrac{3}{10}\)

\(\dfrac{1}{x+1}=\dfrac{1}{3}-\dfrac{3}{10}\)

\(\dfrac{1}{x+1}=\dfrac{1}{30}\)

\(\Rightarrow x+1=30\)

\(x=30-1\)

\(x=29\)

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