Cố gắng lên, mấy thánh sẽ phù hộ con!!!
\(\left(1-\frac{1}{1+2}\right)-\left(1-\frac{1}{1+2+3}\right)-.....-\left(1-\frac{1}{1+2+......+2018}\right)\)
\(C=1-1-1-1....-1-\frac{1}{3}+\frac{1}{1+2+3}+.....+\frac{1}{1+2+.....+2018}\)
\(C=-2017-\frac{1}{3}+\frac{1}{3.4:2}+......+\frac{1}{2019.2018:2}\)
\(\frac{1}{2}C=\frac{1}{3.4}+\frac{1}{4.5}+......+\frac{1}{2019.2018}-1008,5-\frac{1}{6}\)
\(\frac{1}{2}C=\frac{1}{3}-\frac{1}{2019}-1008,5-\frac{1}{6}\)
\(\Rightarrow C=\frac{2}{3}-\frac{2}{2019}-2007-\frac{1}{3}=\frac{1}{3}-\frac{2}{2019}-2007\)
Đến đây tự tính tiếp