Showing convergence and integrability from $\sum_{n=0}^\infty \int_X |f_n| \, d \mu \lt \infty$ I have seen this question here but I didnt find a comprehensible answer so ill try my luck.
Let $(f_n)$ be a sequence of almost everywhere integrable functions defined on X with $$\sum_{n=0}^\infty \int_X |f_n| \, d \mu \lt \infty$$
Show  that $$f(x) := \sum_{n=0}^\infty f_n(x)$$
$1)$ converges for almost all $x$ 
$2)$ the limit function $f$ is integrable
$3) \int_X f \, d\mu = \sum_{n=0}^\infty \int_Xf_n \, d\mu$
My try:
$1)$ Define $g_N := \sum_{n=0}^N|f_n|.$ It follows that $g_N$ is a monotone increasing function so I can use the monotone convergence theorem. 
It follows that : $$\lim_{N \rightarrow \infty}\int_X g_N \, d \mu  = \int_X \lim_{N \rightarrow \infty} g_N \, d \mu $$
$\Longrightarrow \sum_{n=0}^\infty|f_n| \lt \infty$ so the sum converges for almost all $x$
$2)$ Since all $f_n $ are measurable and we showed that $f = \lim f_n$ is finite it follows that $f$ is integrable
$3)$ I would say it strictly follows from dominated convergence theorem with $1)$ and   $2)$.
Any help here is appreciated thanks

Let
$$
F_n= \sum_{k=0}^{n}f_k(x); \quad F(x)= \sum_{k=0}^{\infty}f_k(x)
$$
Clearly, $F_n\in L^1$. Further
$$
|F_n|\leq \sum_{k=0}^{n} |f_k(x)|\leq \sum_{k=0}^{\infty} |f_k(x)|\in  L^1\tag{1}
$$
since
$$
\int_X \sum_{k=0}^{\infty} |f_k(x)|=\sum_{k=0}^{\infty}\int_X |f_k| \,d \mu <\infty
$$
by the Monotone convergence theorem. The inquality in (1) also implies that $F$ is integrable. To see why let $n\to\infty$ and integrate both sides. 
Since $F_n\to F$, the DCT implies that $\int F_n\to \int F$ and hence
$$
\int_X f d\mu = \sum_{n=0}^{\infty}\int_Xf_n\,d\mu
$$
