c2:
ta có [TEX]\hat{A}+\hat{B}+\hat{C}=180^o[/TEX](tổng 3 góc trong 1 tam giác)
mà [TEX]\hat{B}=2\hat{A}[/TEX],[TEX]\hat{C}=4\hat{A}[/TEX]
\Rightarrow [TEX]\hat{A}+2\hat{A}+\hat{A}=180^o[/TEX]
\Rightarrow [TEX]\hat{A}=\frac{\pi}{7}[/TEX],[TEX]\hat{B}=\frac{2\pi}{7}[/TEX],
[TEX]\hat{C}=\frac{4\pi}{7}[/TEX]
Theo định lí hàm số sin ta có:
[TEX]\frac{a}{sin A}=\frac{b}{sin B}=\frac{c}{sin C}=2R[/TEX]
\Rightarrow
[TEX]a=2R.sin.\frac{\pi}{7}[/TEX]
[TEX]b=2R.sin.\frac{2\pi}{7}[/TEX]
[TEX]c=2R.sin.\frac{4\pi}{7}[/TEX]
\Rightarrow
[TEX]\frac{1}{b}+\frac{1}{c}=\frac{1}{2R}.(\frac{1}{sin.\frac{2\pi}{7}}+\frac{1}{sin.\frac{4\pi}{7}})[/TEX]
[TEX]=\frac{1}{2R}.\frac{sin.\frac{4\pi}{7}+sin.\frac{2\pi}{7}}{sin.\frac{4\pi}{7}sin.\frac{2\pi}{7[/TEX]
[TEX]=\frac{1}{2R}.\frac{2.sin.\frac{3\pi}{7}.cos.\frac{2\pi}{7}}{2.sin.\frac{\pi}{7}.cos.\frac{\pi}{7}.sin\frac{4\pi}{7}}[/TEX]
[TEX]=\frac{1}{2R}.\frac{1}{sin.\frac{\pi}{7}[/TEX]
[TEX]=\frac{1}{a}[/TEX]