Xét số hạng tổng quát:
$$\frac{1}{x(x+1)(x+2)}=\frac{1}{2}\frac{(x+2)-x}{x(x+1)(x+2)}=\frac{1}{2}(\frac{1}{x(x+1)}-\frac{1}{(x+1)(x+2)})$$
Áp dụng ta có:
$$B=\frac{1}{2}(\frac{1}{1.2}-\frac{1}{2.3}+\frac{1}{2.3}-\frac{1}{3.4}+...+\frac{1}{199.200}-\frac{1}{200.201})\\=\frac{1}{2}(\frac{1}{1.2}-\frac{1}{200.201})=\frac{20099}{80400}$$