(Chứng minh (*):
sin2A+sin2B+sin2C=1−cos2A2+1−cos2B2+sin2C
=2−12(cos2A+cos2B)−cos2C=2+cosCcos(A−B)−cos2C
=2+cosC[cos(A−B)+cos(A+B)]=2+2cosAcosBcosC)
Thẻ
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