Giải thích các bước giải:
Câu 18:
Ta có :$\dfrac{b}{\sin B}=\dfrac{c}{\sin C}=\dfrac{a}{\sin A}$
$\to \dfrac{b+c}{\sin B+\sin C}=\dfrac{a}{\sin A}$
$\to \dfrac{2a}{\sin B+\sin C}=\dfrac{a}{\sin A}$
$\to \sin B+\sin C=2\sin A\to D$
$\to$
Câu 19:
Ta có :
$\cos\dfrac{A+B+2C}{2}$
$=\cos\dfrac{A+B+C+C}{2}$
$=\cos\dfrac{180^o+C}{2}$
$=\cos(90^o+\dfrac{C}{2})$
$=\sin(90^o-(90^o+\dfrac{C}{2}))$
$=\sin(-\dfrac{C}{2})$
$=-\sin(\dfrac{C}{2})$
$\to D$
