a) Cho A=\(\left(\frac{1}{2}-1\right)\left(\frac{1}{3}-1\right)\left(\frac{1}{4}-1\right)......\left(\frac{1}{200}-1\right)\). Hãy so sánh A với \(-\frac{1}{199}\) b) Tính B= \(1+\frac{1}{2}\left(1+2\right)+\frac{1}{3}\left(1+2+3\right)+.....+\frac{1}{500}\left(1+2+3+...+500\right)\)