Giải thích các bước giải:
Ta có:
$\widehat{DIC}+\widehat{ASB}$
$=(\widehat{IBC}+\widehat{ICB})+(180^o-\widehat{SBA}-\widehat{SAB})$
$=(\widehat{DBC}+\widehat{ACB})+(180^o-\widehat{CBA}-\widehat{DAB})$
$=(\widehat{DBC}+\widehat{ACB})+(180^o-\widehat{DAB})-\widehat{CBA}$
$=\widehat{DBC}+\widehat{ACB}+\widehat{DCB}-\widehat{CBA}$
$=\widehat{ACB}+\widehat{DCB}-(\widehat{CBA}-\widehat{DBC})$
$=\widehat{ACB}+\widehat{DCB}-\widehat{ABD}$
$=\widehat{ACB}+\widehat{DCB}-\widehat{ACD}$
$=\widehat{ACB}+(\widehat{DCB}-\widehat{ACD})$
$=\widehat{ACB}+\widehat{ACB}$
$=2\widehat{ACB}$