1) Xét \Delta PFM \sim \Delta CNI
Suy ra: \dfrac{PF}{CN} = \dfrac{FM}{IN} \to IN.FP = CN.FM
Tương tự ta cũng có: NF.MB = IM.FP = IN.FP
Suy ra: NF.MB = CN.FM
Lại có: \widehat{BMF} = \widehat{CNF}
Suy ra: \Delta FMB \sim \Delta FNC
Suy ra: \widehat{MFB} = \widehat{NFC} \iff 90^o - \widehat{MFB} =...