Giải thích các bước giải:
a.Ta có : $\Diamond DEFG$ có:
$\hat D+\hat E+\hat F+\hat G$
$=\widehat{EDF}+\widehat{FDG}+\hat E+\widehat{EFD}+\widehat{DFG}+\hat G$
$=(\widehat{EDF}+\widehat{DFE}+\widehat{DEF})+(\widehat{FDG}+\widehat{DGF}+\widehat{DFG})$
$=180^o+180^o$
$=360^o$
$\to 2x+3x+4x+x=360^o$
$\to 10x=360^o$
$\to x=36^o$
$\to \hat D=72^o, \hat E=108^o,\hat F=144^o,\hat G=36^o$
b.Gọi $AC\cap BD=H$
$\to AB+CD<(HA+HB)+(HD+HC)=(HA+HC)+(HB+HD)=AC+DB$