Đáp án:
$\widehat{HAD} = 10^\circ$
Giải thích các bước giải:
$\widehat{HAD} = 90^\circ - \widehat{HDA}$
$\to \widehat{HAD} = 90^\circ - (\widehat{DAC} + \widehat{C})$
$\to \widehat{HAD} = 90^\circ - \dfrac{1}{2}\widehat{A} - \widehat{C}$
$\to \widehat{HAD} = 90^\circ - \dfrac{1}{2}(180^\circ - \widehat{B} - \widehat{C}) - \widehat{C}$
$\to \widehat{HAD} = \dfrac{1}{2}\widehat{B} + \dfrac{1}{2}\widehat{C} - \widehat{C}$
$\to \widehat{HAD} = \dfrac{1}{2}(\widehat{B} - \widehat{C})$
$\to \widehat{HAD} = \dfrac{1}{2}\cdot20^\circ$
$\to \widehat{HAD} = 10^\circ$
