tỉ số lượng giác

N

nhuquynhdat

Kẻ phân giác BD

$\Longrightarrow \dfrac{AD}{DC}=\dfrac{AB}{BC} \Longrightarrow \dfrac{AD}{AB}=\dfrac{CD}{BC}=\dfrac{AD+CD}{AB+BC}=\dfrac{AC}{AB+BC}$

$tan \dfrac{B}{2}= tan ABD= \dfrac{AD}{AB}=\dfrac{AC}{AB+BC}$
 
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