Cau5
SinA=(sinB+sinC) /(cosB+cosC) suy ra [tex]\frac{a}{2R}=\frac{b/2R+c/2R}{\frac{a^2+c^2-b^2}{2ac}+\frac{a^2+b^2-c^2}{2ab}}[/tex] suy ra a=[tex]\frac{(b+c) 2abc}{(b+c) (a^2-b^2-c^2+2bc) }[/tex] suy ra 2bc=2bc+a^2-b^2-c^2 suy ra a^2=b^2+c^2 suy ra dpcm