Vì `c⊥a,c⊥b`
$⇒a//b$ (từ vuông góc đến song song)
$⇒\widehat{C_1}=\widehat{D_2}=125^\circ$ ($2$ góc so le trong)
Ta có: $\widehat{D_2}+\widehat{D_1}=180^\circ$ ($2$ góc kề bù)
Mà $\widehat{D_2}=125^\circ$
$⇒125^\circ+\widehat{D_1}=180^\circ$
$⇒\widehat{D_1}=180^\circ-125^\circ=55^\circ$
