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Ta có: *)A.5=5(1/3.8+1/8.13+...+1/33.38)
=5/3.8+5/8.13+...+5/33.38
=1/3-1/8+1/8-1/13+...+1/33-1/38
=1/3-1/38
=> A=(1/3-1/38).1/5
*)7B=7/3.10+7/10.17+7/17.24+...+7/31.38
=1/3-1/10+1/10-1/17+...+1/31-1/38
=1/3-1/38
=>B=(1/3-1/38).1/7
Do đó a/b=(1/5)/(1/7)=7/5
k mk nha!
a) \(27^{64}:81^{20}=3^{192}:3^{80}=3^{112}\)
b) \(\left(\dfrac{1}{8}\right)^{20}:\left(\dfrac{1}{16}\right)^9=\left(\dfrac{1}{2}\right)^{60}:\left(\dfrac{1}{2}\right)^{36}=\left(\dfrac{1}{2}\right)^{24}\)
c) \(\dfrac{1}{3}:\dfrac{1}{5}-\dfrac{1}{6}=\dfrac{5}{3}-\dfrac{1}{6}=\dfrac{10}{6}-\dfrac{1}{6}=\dfrac{9}{6}=\dfrac{3}{2}\)
\(a)\frac{x}{4}-\frac{3}{7}+\frac{2}{5}=\frac{31}{140}\)
\(\Leftrightarrow\frac{x}{4}-\frac{1}{35}=\frac{31}{140}\)
\(\Leftrightarrow\frac{x}{4}=\frac{1}{4}\Leftrightarrow x=1\)
\(b)\frac{5}{12}+\frac{5}{x}-\frac{1}{8}=\frac{1}{12}\)
\(\Leftrightarrow\frac{5}{x}+\frac{7}{24}=\frac{1}{12}\)
\(\Leftrightarrow\frac{5}{x}=-\frac{5}{24}\Leftrightarrow x=-24\)