=3/1-3/4+3/4-3/7+3/7-3/10+3/10-3/13
=1/1-1/4+1/4-1/7+1/7-1/10+1/10-1/13
=1-1/13
=12/13
3/1x4+3/4x7+3/7x10+3/10x13
=1-1/4+1/4-1/7+1/7-1/10+1/10-1/13
=1-1/13
=12/13
\(\frac{3}{1\times4}+\frac{3}{4\times7}+\frac{3}{7\times10}+\frac{3}{10\times13}\)
\(=3\times\left(\frac{1}{1\times4}+\frac{1}{4\times7}+\frac{1}{7\times10}+\frac{1}{10\times13}\right)\)
\(=3\times\left(1-\frac{1}{4}+\frac{1}{4}-\frac{1}{7}+\frac{1}{7}-\frac{1}{10}+\frac{1}{10}-\frac{1}{13}\right)\)
\(=3\times\left(1-\frac{1}{13}\right)\)
\(=3\times\frac{12}{13}\)
\(=\frac{36}{13}\)
Thằng Trần Hoàng Việt lm sai rồi dơ mặt ban lãnh đạo CTV
\(\frac{3}{1.4}+\frac{3}{4.7}+...+\frac{3}{10.13}\)
\(=1-\frac{1}{4}+\frac{1}{4}-\frac{1}{7}+..+\frac{1}{10}-\frac{1}{13}\)
\(=1-\frac{1}{13}=\frac{12}{13}\)
\(\frac{3}{1\cdot4}+\frac{3}{4\cdot7}+\frac{4}{7\cdot10}+\frac{3}{10\cdot13}\)
\(=\frac{1}{1\cdot4}+\frac{1}{4\cdot7}+\frac{1}{7\cdot10}+\frac{1}{10\cdot13}\)
\(=\frac{1}{1}-\frac{1}{4}+\frac{1}{4}-\frac{1}{7}+\frac{1}{7}-\frac{1}{10}+\frac{1}{10}-\frac{1}{13}\)
\(=\frac{1}{1}-\frac{1}{13}\)
\(=\frac{13}{13}-\frac{1}{13}\)
\(=\frac{12}{13}\)