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\(\dfrac{1}{3.5}+\dfrac{1}{5.7}+\dfrac{1}{7.9}+...+\dfrac{1}{203.205}\)
\(=\dfrac{1}{2}.\left(\dfrac{2}{3.5}+\dfrac{2}{5.7}+\dfrac{2}{7.9}+...+\dfrac{2}{203.205}\right)\)
\(=\dfrac{1}{2}.\left(\dfrac{1}{3}-\dfrac{1}{5}+\dfrac{1}{5}-\dfrac{1}{7}+\dfrac{1}{7}-\dfrac{1}{9}+...+\dfrac{1}{203}-\dfrac{1}{205}\right)\)
\(=\dfrac{1}{2}.\left(\dfrac{1}{3}-\dfrac{1}{205}\right)\)
\(=\dfrac{1}{2}.\dfrac{202}{615}\)
\(=\dfrac{101}{615}\)
Chúc bạn học tốt!
\(2B=2\left(\frac{1}{3}-\frac{1}{5}+\frac{1}{5}-\frac{1}{7}+...+\frac{1}{23}-\frac{1}{25}\right)\)
\(2B=2\left(\frac{1}{3}-\frac{1}{25}\right)\)
\(2B=2\times\frac{22}{75}\)
\(B=\frac{44}{75}\)
a,\(\frac{2}{3.5}+\frac{2}{5.7}+.......+\frac{2}{11.13}\)
=\(\frac{1}{3}-\frac{1}{5}+\frac{1}{5}-\frac{1}{7}+.............+\frac{1}{11}-\frac{1}{13}\)
=\(\frac{1}{3}-\frac{1}{13}\)
=\(\frac{10}{39}\)
b,Đặt A=\(\frac{1}{1.2.3.4}+\frac{1}{2.3.4.5}+.............+\frac{1}{27.28.29.30}\)
3A=\(\frac{3}{1.2.3.4}+\frac{3}{2.3.4.5}+...........+\frac{3}{27.28.29.30}\)
3A=\(\frac{1}{1.2.3}-\frac{1}{2.3.4}+\frac{1}{2.3.4}-\frac{1}{3.4.5}+.............+\frac{1}{27.28.29}-\frac{1}{28.29.30}\)
3A=\(\frac{1}{1.2.3}-\frac{1}{28.29.30}\)
3A=\(\frac{1}{6}-\frac{1}{24360}\)
3A=\(\frac{1353}{8120}\)
A=\(\frac{451}{8120}\)
\(A=\frac{1}{3.5}+\frac{1}{5.7}+\frac{1}{7.9}+...+\frac{1}{99.101}\)
\(\Rightarrow2A=\frac{2}{3.5}+\frac{2}{5.7}+\frac{2}{7.9}...+\frac{2}{99.101}\)
\(\Rightarrow2A=\frac{1}{3}-\frac{1}{5}+\frac{1}{5}-\frac{1}{7}+\frac{1}{7}-\frac{1}{9}+...+\frac{1}{99}-\frac{1}{101}\)
\(\Rightarrow2A=\frac{1}{3}-\frac{1}{101}\)
\(\Rightarrow2A=\frac{101}{303}-\frac{3}{303}\)
\(\Rightarrow2A=\frac{98}{303}\)
\(\Rightarrow A=\frac{98}{303}:2=\frac{98}{303.2}=\frac{98}{606}=\frac{49}{303}\)
lên 820 điểm hỏi đáp nha
1: \(=\dfrac{1}{2}\left(\dfrac{1}{3}-\dfrac{1}{5}+\dfrac{1}{5}-\dfrac{1}{7}+...+\dfrac{1}{11}-\dfrac{1}{13}\right)\)
=1/2*10/39
=5/39
2: \(=\dfrac{5}{2}\left(1-\dfrac{1}{3}+\dfrac{1}{3}-\dfrac{1}{5}+...+\dfrac{1}{9}-\dfrac{1}{11}\right)=\dfrac{5}{2}\cdot\dfrac{10}{11}=\dfrac{50}{22}=\dfrac{25}{11}\)