Giải thích các bước giải:

Ta có:

$\widehat{BID}$

$=180^o-\hat A-\widehat{ABI}-\widehat{ADI}$

$=180^o-\hat A-\dfrac12\hat B-\dfrac12\hat D$

$=180^o-\hat A-\dfrac12(\hat B+\hat D)$

$=180^o-\hat A-\dfrac12(180^o-\hat A-\hat C)$

$=180^o-\hat A-90^o+\dfrac12\hat A+\dfrac12\hat C$

$=90^o-\dfrac12\hat A+\dfrac12\hat C$

$=90^o-\dfrac12(\hat A-\hat C)$

$=90^o-\dfrac12\cdot 60^o$

$=90^o-30^o$

$=60^o$