Xét tứ giác ABCD có:
$\widehat{A} + \widehat{B} + \widehat{C} + \widehat{D}=360^o$
Hay $70^o + 90^o + \widehat{C} + \widehat{D}=360^o$
$⇒\widehat{C} + \widehat{D}= 360^o - (70^o+90^o)= 360^o - 160^o = 200^o$
Hay $\widehat{C_1} + \widehat{C_2}+\widehat{D_1} + \widehat{D_2}= 200^o$
$⇒\widehat{C_1} + \widehat{D_1}= \dfrac{1}{2}\widehat{C} + \widehat{D}=\dfrac{1}{2}200^o=100^o$
Xét $\Delta{OCD}$ có:
$\widehat{C_1} + \widehat{D_1}+\widehat{COD}=180^o$ (tổng 3 góc $1 \Delta$)
hay $100^o + \widehat{COD}=180^o$
$⇒ \widehat{COD}=80^o$
*Hình vẽ: ẢNH
