=15/30+5/30+1/12+1/20+1/30+1/42
=2/3+1/12+1/20+1/30+1/42
=8/12+1/12+1/20+1/30+1/42
=3/4+1/20+1/30+1/42
=15/20+1/20+1/30+1/42
=4/5+1/30+1/42
=24/30+1/30+1/42
=5/6+1/42
=35/42+1/42
=6/7
A=\(\frac{1}{2}+\frac{1}{6}+\frac{1}{12}+\frac{1}{20}+\frac{1}{30}+\frac{1}{42}\)
A=\(\frac{1}{1\cdot2}+\frac{1}{2\cdot3}+\frac{1}{3\cdot4}+\frac{1}{4\cdot5}+\frac{1}{5\cdot6}+\frac{1}{6\cdot7}\)
A=\(1-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+\frac{1}{4}-\frac{1}{5}+\frac{1}{5}-\frac{1}{6}+\frac{1}{6}-\frac{1}{7}\)
A=\(1-\frac{1}{7}\)
A=\(\frac{6}{7}\)
ngọc ơi đây là tính nhanh sai phân tính như vũ ấy
