Let $H$ and $K$ be normal subgroups of $G$, if $H \cap K = \{e\}$ prove that $hk = kh$ for all $h \in H, k \in K$
$H_1 ,H_2 \unlhd \, G$ with $H_1 \cap H_2 = \{1_G\} $. Prove every two elements in $H_1, H_2$ commute