Ta có:
$\widehat{A}$ + $\widehat{B}$ + $\widehat{C}$ + $\widehat{D}$ = $360^{O}$
⇒ $70^{O}$ + $100^{O}$ + $\widehat{C}$ + $\widehat{D}$ = $360^{O}$
⇒ $170^{O}$ + $\widehat{C}$ + $\widehat{D}$ = $360^{O}$
⇒ $\widehat{C}$ + $\widehat{D}$ = $360^{O}$ - $170^{O}$
⇒ $\widehat{C}$ + $\widehat{D}$ = $90^{O}$
⇒ $\frac{\widehat{C} + \widehat{D}}{2}$ = $45^{O}$
⇒ $\frac{\widehat{C}}{2}$ + $\frac{\widehat{D}}{2}$ = $45^{O}$
⇒ $\widehat{OCD}$ + $\widehat{ODC}$ = $45^{O}$ (TÍNH CHẤT PHÂN GIÁC)
⇒ $\widehat{COD}$ = $180^{O}$ - $45^{O}$ ( $\widehat{COD}$ + $\widehat{OCD}$ + $\widehat{ODC}$ = $180^{O}$)
⇒ $\widehat{COD}$ = $135^{O}$
