S=S_{\triangle ABC}+S_{\widehat{AC}}+S_{\widehat{BC}}=\frac 1 2AC\cdot BC\sin\angle ACB+S_{\widehat{AC}}+S_{\widehat{BC}}<\frac 1 2AC\cdot BC+S_{\widehat{AC}}+S_{\widehat{BC}}=S_{\triangle AB'C}+S_{\widehat{AC}}+S_{\widehat{B'C}}