Chứng minh rằng với mọi m ∈ N ta có

$$\begin{gathered} Bài 15 Chứng minh rằng với mọi m\in N ta có \\ a) \frac{4}{4m+2}=\frac{1}{m+1}+\frac{1}{(m+1)(2m+1)} \\ b) \frac{4}{4m+3}=\frac{1}{m+2}+\frac{1}{(m+1)(m+2)}+\frac{1}{(m+1)(4m+3)} \\ c)\frac{4}{8m+5}=\frac{1}{2(m+1)}+\frac{1}{2(m+1)(3m+2)}+\frac{1}{2(3m+2)(8m+5)} \\ d)\frac{4}{3m+2}=\frac{1}{m+1}+\frac{1}{3m+2}+\frac{1}{(m+1)(3m+2)} \end{gathered}$$