Vì $\widehat{BCE}$ và $\widehat{ECF}$ là `2` góc kề bù
`⇒` $\widehat{BCE}$ `+` $\widehat{ECF}$ `=` $180^{o}$
Hay $90^{o}$ `+` $\widehat{ECF}$ `=` $180^{o}$
`=>`$\widehat{ECF}$ `=` $180^{o}$ `-` $90^{o}$
`=>`$\widehat{ECF}$ `=` $90^{o}$
Ta có: $\widehat{ECF}$ `+` $\widehat{CEF}$ `+` $\widehat{CFE}$ `=` $180^{o}$
Hay $90^{o}$ `+` $55^{o}$ `+` $\widehat{CFE}$ `=` $180^{o}$
`=>`$145^{o}$ `+` $\widehat{CFE}$ `=` $180^{o}$
`=>`$\widehat{CFE}$ `=` $180^{o}$ `-` $145^{o}$
`=>`$\widehat{CFE}$ `=` $35^{o}$
Vậy $\widehat{CFE}$ `=` $35^{o}$