[ Toán 11] BT cấp số cộng

T

truongduong9083

Gợi ý:
Theo giả thiết ta có
$sinA.sinB=\dfrac{3sin^2\frac{C}{2}.cos^2\frac{C}{2}}{cos^2\frac{C}{2}}$
$\Leftrightarrow sinA.sinB=\dfrac{3sin^2C}{2(1+cosC)}$
$\Leftrightarrow \dfrac{ab}{4R^2} = \dfrac{3c^2}{8R^2(1+\frac{a^2+b^2-c^2}{2ab})}$
$\Leftrightarrow a+b = 2c$
 
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