2 h 30 chiều nay mình phả đi học rồi , làm ơn giúp mình vói mình xin cảm ơn và sẽ k
\(a,\frac{3}{20}+\frac{3}{20}+\frac{3}{42}+...+\frac{3}{132}\)
\(=3\left(\frac{1}{4\cdot5}+\frac{1}{5\cdot6}+\frac{1}{6\cdot7}+...+\frac{1}{11\cdot12}\right)\)
\(=3\left(\frac{1}{4}-\frac{1}{5}+\frac{1}{5}-\frac{1}{6}+\frac{1}{6}-\frac{1}{7}+...+\frac{1}{11}-\frac{1}{12}\right)\)
\(=3\left(\frac{1}{4}-\frac{1}{12}\right)\)
\(=3\cdot\frac{1}{6}=\frac{1}{2}\)
a) \(\frac{3}{20}+\frac{3}{30}+\frac{3}{42}+...+\frac{3}{132}\)
= \(3\times\left(\frac{1}{20}+\frac{1}{30}+\frac{1}{42}+...+\frac{1}{132}\right)\)
= \(3\times\left(\frac{1}{4\times5}+\frac{1}{5\times6}+\frac{1}{6\times7}+...+\frac{1}{11\times12}\right)\)
= \(3\times\left(\frac{1}{4}-\frac{1}{5}+\frac{1}{5}-\frac{1}{6}+\frac{1}{6}-\frac{1}{7}+...+\frac{1}{11}-\frac{1}{12}\right)\)
= \(3\times\left(\frac{1}{4}-\frac{1}{12}\right)\)
= \(3\times\frac{1}{6}\)
= \(\frac{1}{2}\)
