\(E=\frac{2}{1.2.3}+\frac{2}{2.3.4}+...+\frac{2}{98.99.100}\)
\(E=\frac{1}{1.2}-\frac{1}{2.3}+\frac{1}{2.3}-\frac{1}{3.4}+...+\frac{1}{98.99}-\frac{1}{99.100}\)
\(E=\frac{1}{1.2}-\frac{1}{99.100}\)
rồi đấy tự tính tiếp
2E=\(\frac{1}{1.2.3}\)+\(\frac{1}{2.3.4}\)+........+\(\frac{1}{98.99.100}\)
2E = \(\frac{1}{1.2}\)-\(\frac{1}{2.3}\)+ \(\frac{1}{2.3}\)-\(\frac{1}{3.4}\)+.........+\(\frac{1}{98.99}\)-\(\frac{1}{99.100}\)
2E = \(\frac{1}{1.2}\)- \(\frac{1}{99.100}\)
2E = \(\frac{4949}{9900}\)
E = \(\frac{4949}{9900}\)\(:2\)
E = \(\frac{4949}{19800}\)