Let $f_1$ and $f_2$ be bounded functions on $[a, b]$. Prove that $L(f_1) + L(f_2) \leq L(f_1 + f_2)$ (Darboux intergral)
Let $f_1 , f_2: I\mapsto \mathbb{R}$ bounded functions. Show that $L(f_1)+L(f_2)\leq L(f_1+f_2)$ (Riemann integral)