nhan dang tam giac

N

niemkieuloveahbu

$$ \frac{b}{cosB}+\frac{c}{cosC}=\frac{a}{sinBsinC}\\ Ap\ dung\ dinh\ li \ sin:\\ \frac{sinB}{cosB}+\frac{sinC}{cosC}=\frac{sinA}{sinBsinC}\\ \Leftrightarrow \frac{sinBcosC+sinCcosB}{cosBcosC}=\frac{sinA}{sinBcosB}\\ \Leftrightarrow \frac{sin(B+C)}{cosBcosC}=\frac{sinA}{sinBsinC}\\ \Leftrightarrow \frac{sinA}{cosBcosC}=\frac{sinA}{sinBsinC}(sin(B+C)=sin(\pi -A)=sin A)$$
Tự làm tiếp nhé.
 
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