\(\frac{1}{1.2.3}+\frac{1}{2.3.4}+\frac{1}{3.4.5}+...+\frac{1}{14.15.16}\)
\(=\frac{1}{2}.\left(\frac{2}{1.2.3}+\frac{2}{2.3.4}+\frac{2}{3.4.5}+...+\frac{2}{14.15.16}\right)\)
\(=\frac{1}{2}.\left(\frac{1}{1.2}-\frac{1}{2.3}+\frac{1}{2.3}-\frac{1}{3.4}+\frac{1}{3.4}-\frac{1}{4.5}+...+\frac{1}{14.15}-\frac{1}{15.16}\right)\)
\(=\frac{1}{2}.\left(\frac{1}{1.2}-\frac{1}{15.16}\right)\)
\(=\frac{1}{2}.\left(\frac{1}{2}-\frac{1}{240}\right)\)
\(=\frac{1}{2}.\frac{119}{240}\)
\(=\frac{119}{480}\)
Bài làm:
Ta có:\(\frac{1}{1.2.3}+\frac{1}{2.3.4}+\frac{1}{3.4.5}+...+\frac{1}{14.15.16}\)
\(=\frac{1}{2}\left(\frac{2}{1.2.3}+\frac{2}{2.3.4}+\frac{2}{3.4.5}+...+\frac{2}{14.15.16}\right)\)
\(=\frac{1}{2}\left(\frac{1}{1.2}-\frac{1}{2.3}+\frac{1}{2.3}-\frac{1}{3.4}+...+\frac{1}{14.15}-\frac{1}{15.16}\right)\)
\(=\frac{1}{2}\left(\frac{1}{1.2}-\frac{1}{15.16}\right)\)
\(=\frac{1}{2}\left(\frac{1}{2}-\frac{1}{240}\right)\)
\(=\frac{1}{2}.\frac{119}{240}=\frac{119}{480}\)
Có:\(\frac{1}{1.2.3}+\frac{1}{2.3.4}+...+\frac{1}{14.15.16}\)
\(=\frac{1}{2}\left(\frac{2}{1.2.3}+\frac{2}{2.3.4}+...+\frac{2}{14.15.16}\right)\)
\(=\frac{1}{2}\left(\frac{1}{1.2}-\frac{1}{2.3}+\frac{1}{2.3}-\frac{1}{3.4}+...+\frac{1}{14.15}-\frac{1}{15.16}\right)\)
\(=\frac{1}{2}\left(\frac{1}{1.2}-\frac{1}{15.16}\right)\)
\(=\frac{1}{2}\left(\frac{1}{2}-\frac{1}{240}\right)\)
\(=\frac{1}{2}\times\frac{119}{240}=\frac{119}{480}\)
\(\frac{1}{1.2.3}+\frac{1}{2.3.4}+...+\frac{1}{14.15.16}\)
\(=\frac{1}{2}\left(\frac{2}{1.2.3}+\frac{2}{2.3.4}+...+\frac{2}{14.15.16}\right)\)
\(=\frac{1}{2}\left(\frac{1}{1.2}-\frac{1}{2.3}+\frac{1}{2.3}-\frac{1}{3.4}+...+\frac{1}{14.15}-\frac{1}{15.16}\right)\)
\(=\left(\frac{1}{1.2}-\frac{1}{15.16}\right)\)
\(=\frac{1}{2}\)
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