Ta có: tổng bốn góc trong 1 tứ giác có số đo là: `360°.`
$⇔\widehat{C}+\widehat{D}+\widehat{E}+\widehat{F}=360°$
Mà lại có: $\widehat{C}=\widehat{D}=\widehat{E}=\widehat{F}$
`⇒` $\widehat{C}+\widehat{D}+\widehat{E}+\widehat{F}=360°$
`⇔` $4\widehat{C}=4\widehat{D}=4\widehat{E}=4\widehat{F}=360°$
`⇔ `$\widehat{C}=\widehat{D}=\widehat{E}=\widehat{F}=360:4=90°$
Vậy $\widehat{C}=\widehat{D}=\widehat{E}=\widehat{F}=360:4=90°.$