giup minh voi mn oi

A=  \(\frac{221}{210}\)

k tui nha, hậu tạ

sao mày o làm nhanh ả

không nói tục chửi bậy nha

=35/6

=35/6

35/6

\(=\dfrac{1}{2}-\dfrac{1}{3}+\dfrac{1}{3}-\dfrac{1}{4}+...+\dfrac{1}{6}-\dfrac{1}{7}=\dfrac{1}{2}-\dfrac{1}{7}=\dfrac{5}{14}\)

\(=\dfrac{1}{2}-\dfrac{1}{3}+\dfrac{1}{3}-\dfrac{1}{4}+...+\dfrac{1}{20}-\dfrac{1}{21}=\dfrac{21-2}{42}=\dfrac{19}{42}\)

Lời giải:
Gọi biểu thức số 1 là A và số 2 là B

\(A=\frac{3-2}{2\times 3}+\frac{4-3}{3\times 4}+\frac{5-4}{4\times 5}+\frac{6-5}{5\times 6}+\frac{7-6}{6\times 7}\)

\(=\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+\frac{1}{4}-\frac{1}{5}+\frac{1}{5}-\frac{1}{6}+\frac{1}{6}-\frac{1}{7}\)

\(=\frac{1}{2}-\frac{1}{7}=\frac{5}{14}\)

B tương tự A:
\(B=\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+\frac{1}{4}-\frac{1}{5}+....+\frac{1}{20}-\frac{1}{21}\)

\(=\frac{1}{2}-\frac{1}{21}=\frac{19}{42}\)

9X6+6:6-6X7

=54 +1 -42

=55-42

=13

9x6+6:6-6x7

= 54 + 1 - 42

= 55 - 42

= 13

9X6+6:6-6X7=54+2-42=13

đáp số: 13

A = \(\dfrac{3}{5\times6}\) + \(\dfrac{3}{6\times7}\)+...+ \(\dfrac{3}{87\times88}\)

A = 3\(\times\)( \(\dfrac{1}{5\times6}\) + \(\dfrac{1}{6\times7}\)+...+ \(\dfrac{1}{87\times88}\))

A = 3 \(\times\)( \(\dfrac{1}{5}\) - \(\dfrac{1}{6}\) + \(\dfrac{1}{6}\) - \(\dfrac{1}{7}\) +...+ \(\dfrac{1}{87}\) - \(\dfrac{1}{88}\))

A = 3 \(\times\)( \(\dfrac{1}{5}\) - \(\dfrac{1}{88}\))

A = 3 \(\times\) \(\dfrac{83}{440}\)

A = \(\dfrac{249}{440}\)