a) Có $\widehat{A}+\widehat{B}+\widehat{C}=180^{o}\Rightarrow \widehat{A}+\widehat{B}=180^{o}-\widehat{C}$$\Rightarrow \cos (A+B)=\cos (180^{o}-C)=\cos C$
a) Có $\widehat{A}+\widehat{B}+\widehat{C}=180^{o}\Rightarrow \widehat{A}+\widehat{B}=180^{o}-\widehat{C}$$\Rightarrow \cos (A+B)=\cos (180^{o}-C)=-\cos C$
a) Có $\widehat{A}+\widehat{B}+\widehat{C}=180^{o}\Rightarrow \widehat{A}+\widehat{B}=180^{o}-\widehat{C}$$\Rightarrow \cos (A+B)=\cos (180^{o}-C)=\cos C$