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\(A=\frac{1}{3}+\frac{1}{3^2}+...+\frac{1}{3^6}\)
\(3A=1+\frac{1}{3}+...+\frac{1}{3^5}\)
\(3A-A=\left(1+\frac{1}{3}+...+\frac{1}{3^5}\right)-\left(\frac{1}{3}+\frac{1}{3^2}+...+\frac{1}{3^6}\right)\)
\(2A=1-\frac{1}{3^6}=\frac{3^6-1}{3^6}=\frac{728}{729}\)
\(\Rightarrow A=\frac{728}{729}:2=\frac{364}{729}\)
Mong các bạn giúp tớ, tớ sẽ k cho, cảm ơn các bạn.......ek
a, \(A=3a.2.b-a.432b-4ab\)
\(=6ab-432ab-4ab=-430ab\)
b, \(A=-430ab=\left(-430\right).\frac{1}{229}.\frac{1}{433}=\frac{-430}{229.433}\)
3A=\(1+\frac{1}{3}+\frac{1}{3^2}+\frac{1}{3^3}+\frac{1}{3^4}+\frac{1}{3^5}\)
3A-A=\(1-\frac{1}{3^6}\)
2A=\(\frac{3^6-1}{3^6}\)
A=\(\frac{\frac{3^6-1}{3^6}}{2}\)
A=\(\frac{364}{729}\)
3A= 3.(1/2+1/3^2+1/3^3+...+1/3^6)
3A= 1+1/3+1/3^2+1/3^3+...+1/3^5
3A-A=(1+1/3+1/3^2+...+1/3^5)-(1/3+1/3^2+..+1/3^6)
2A=1-1/3^6
2A=1-1/729
2A=728/729
A=364/729
k nhé
A =\(\frac{1}{1+2}\)+\(\frac{1}{1+2+3}\)+...+\(\frac{1}{1+2+3+4...+2014}\)
\(\Rightarrow A=\frac{1}{3}+\frac{1}{6}+\frac{1}{12}+...+\frac{1}{2029105}\)
\(\Rightarrow2A=2\left(\frac{1}{3}+\frac{1}{6}+\frac{1}{12}+...+\frac{1}{2029105}\right)\)
\(\Rightarrow2A=\frac{1}{6}+\frac{1}{12}+\frac{1}{20}+...+\frac{1}{4058210}\)
\(\Rightarrow2A=\frac{1}{2.3}+\frac{1}{3.4}+\frac{1}{4.5}+...+\frac{1}{2014.2015}\)
\(\Rightarrow2A=\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+...+\frac{1}{2014}-\frac{1}{2015}\)
\(\Rightarrow2A=\frac{1}{2}-\frac{1}{2015}\)
\(\Rightarrow2A=\frac{2013}{4030}\)
\(\Rightarrow A=\frac{2013}{8060}\)
ngài Kiệt ღ ๖ۣۜLý๖ۣۜ đúng là không ái sánh bằng sự gian xảo này
\(A=-1,6:\left(1+\frac{2}{3}\right)\)
\(A=\frac{-8}{5}:\frac{5}{3}=-\frac{8}{5}.\frac{3}{5}=\frac{-24}{25}\)
\(B=\frac{7}{5}.\frac{15}{29}-\left(\frac{4}{5}+\frac{2}{3}\right):\frac{11}{5}\)
\(B=\frac{21}{29}-\left(\frac{12}{15}+\frac{10}{15}\right).\frac{5}{11}=\frac{21}{29}-\frac{22}{15}.\frac{5}{11}=\frac{21}{29}-\frac{2}{3}\)
\(B=\frac{63}{87}-\frac{58}{87}=\frac{5}{87}\)
\(A=\frac{1}{3.5}+\frac{1}{5.7}+\frac{1}{7.9}+...+\frac{1}{43.45}\)
\(2A=\frac{2}{3.5}+\frac{2}{5.7}+\frac{2}{7.9}+...+\frac{2}{43.45}\)
\(=\frac{1}{3}-\frac{1}{5}+\frac{1}{5}-\frac{1}{7}+\frac{1}{7}-\frac{1}{9}+...+\frac{1}{43}-\frac{1}{45}\)
\(=\frac{1}{3}-\frac{1}{45}=\frac{15}{45}-\frac{1}{45}=\frac{14}{45}\)
\(\Rightarrow A=\frac{14}{45}:2=\frac{14}{90}=\frac{7}{45}\)
Vậy \(A=\frac{7}{45}\).
Áp dụng công thức : \(\frac{1}{a}-\frac{1}{a+n}=\frac{n}{a\left(a+n\right)}\)
\(A=\frac{1}{3\cdot5}+\frac{1}{5\cdot7}+\frac{1}{7\cdot9}+...+\frac{1}{43\cdot45}\)
\(A=\frac{1}{2}\cdot\left(\frac{1}{3}-\frac{1}{5}+\frac{1}{5}-\frac{1}{7}+\frac{1}{7}-\frac{1}{9}+...+\frac{1}{43}-\frac{1}{45}\right)\)
\(A=\frac{1}{2}\cdot\left(\frac{1}{3}-\frac{1}{45}\right)\)
\(A=\frac{1}{2}\cdot\frac{14}{45}=\frac{7}{45}\)
Bn tính máy tính là ra