[Toán 10] Lượng giác

T

truongduong9083

Chào bạn

Ta có
$$sinA+sinB+sinC = 2sin\frac{A+B}{2}.cos\frac{A-B}{2}+2sin\frac{C}{2}.cos\frac{C}{2}$$
$$= 2cos\frac{C}{2}.cos\frac{A-B}{2}+2sin\frac{C}{2}.cos\frac{C}{2}$$
$$=2cos\frac{C}{2}(cos\frac{A-B}{2}+cos\frac{A+B}{2})$$
$$ = 4cos\frac{A}{2}.cos\frac{B}{2}.cos\frac{C}{2}$$
mới đúng nhé
 
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