Tính các tổng

$$\begin{gathered} Bài 13 Tính các tổng \\ a) A=\frac{a}{(a-b)(a-c)}+\frac{b}{(b-a)(b-c)}+\frac{c}{(c-a)(c-b)} \\ b) B= \frac{a^2}{(a-b)(a-c)}+\frac{b^2}{(b-a)(b-c)}+\frac{c^2}{(c-a)(c-b)} \\ c) C= \frac{1}{1.2}+\frac{1}{2.3}+\frac{1}{3.4}+...+\frac{1}{n(n+1)} HƯỚNG DẪN \frac{1}{k(k+1)}=\frac{1}{k}-\frac{1}{k+1} \\ d) D= +\frac{1}{1.2.3}+\frac{1}{2.3.4}+\frac{1}{3.4.5}+...+\frac{1}{n(n+1)(n+2)} HƯỚNG DẪN \frac{1}{k(k+1)(k=2)}=\frac{1}{2}\left(\frac{1}{k}+\frac{1}{k+2}\right)-\frac{1}{k+1} \end{gathered}$$