Câu a,b,c) Có $\widehat{A}+\widehat{B}=\pi-\widehat{C}$$\Rightarrow sin(A+B)=sin(\pi-C)=sinC$$\Rightarrow cos(A+B)=cos(\pi-C)=-cosC$$\Rightarrow sin(\frac{A+B}2)=sin(\frac{\pi}2-\frac{C}2)=cos(\frac{C}2)$
Câu a,b,c) Có $\widehat{A}+\widehat{B}=\pi-\widehat{C}
\\\Rightarrow
\sin(A+B)=
\sin(\pi-C)=
\sin
C
\\\Rightarrow
\cos(A+B)=
\cos(\pi-C)=-
\cos
C
\\\Rightarrow
\sin
\left(\frac{A+B}2
\right)=
\sin
\left(\
dfrac{\pi}2-\
dfrac{C}2
\right)=
\cos\
dfrac{C}2$