ta có: \frac{a}{sinA}=\frac{b}{sinB}=\frac{c}sinC}\Rightarrow sinB=bsinA/a sinC=csinA/a ta có:sinB+sinC=2sinA\Leftrightarrow bsinA/a+csinA/a=2sinA \Leftrightarrow bsinA+csinA=2asinA\Leftrightarrow b+c=2a(sinA\neq 0)VẬY SINB+SINC=2SINA
ta có:
$\frac{a}{sinA}=\frac{b}{sinB}=\frac{c}
{\sin
C}
$$\Rightarrow sinB=bsinA/a
$ $sinC=csinA/a
$ $ta có:sinB+sinC=2sinA
$$\Leftrightarrow bsinA/a+csinA/a=2sinA
$ $\Leftrightarrow bsinA+csinA=2asinA
$$\Leftrightarrow b+c=2a(sinA\neq 0)
$VẬY SINB+SINC=2SINA