Ai có giúp em 5 câu này được không?
1. A= 2/1.3 + 2/3.5 + .... + 2/49.51
2. A= 10 - /-11/ + 12 - /-13/ + ..... - /-2013/ + 2014
3. B= 2/11.12.13 + 2/12.13.14 + ..... + 2/28.29.30
4. C= 2/3 + 2/9 + 2/27 + 2/81 + 2/243 + 2/729
5. F= 3/15.18 + 11/18.29 + 12/29.41 + 70/41.111 + 1920/111.2013
\(A=\frac{2}{1.3}+\frac{2}{3.5}+\frac{2}{5.7}+.....+\frac{2}{49.50}\)
\(=1-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+.....+\frac{1}{49}-\frac{1}{50}\)
\(=1-\frac{1}{50}\)
\(=\frac{49}{50}\)
5,có thể bn sai đề chỗ \(\frac{1920}{111.2013}\),phải là \(\frac{1902}{111.2013}\)
\(F=\frac{3}{15.18}+\frac{11}{18.29}+\frac{12}{29.41}+\frac{70}{41.111}+\frac{1902}{111.2013}\)
\(=\frac{18-15}{15.18}+\frac{29-18}{18.29}+\frac{41-29}{12}+\frac{111-41}{41.111}+\frac{2013-111}{1902}\)
\(=\frac{1}{15}-\frac{1}{18}+\frac{1}{18}-\frac{1}{29}+\frac{1}{29}-\frac{1}{41}+\frac{1}{41}-\frac{1}{111}+\frac{1}{111}-\frac{1}{2013}\)
\(=\frac{1}{15}-\frac{1}{2013}=\frac{222}{3355}\)