A+B+C=π2sin2A+sin2B+sin2C=1−cos2A2+1−cos2B2+1−cos2C2
=12(3−(cos2A+cos2B+cos2C))
=12(3−(2cos(A+B)cos(A−B)+cos(π−2(A+B))))
=12(3−(2cos(A+B)cos(A−B)−cos2(A+B)))
=12(3−2cos(A+B)cos(A−B)−2cos2(A+B)−1)
=12(2−2cos(A+B)(cos(A−B)−cos(A+B)))
=1−2sinAsinBsinC
cos2A+cos2B+cos2C=cos2A+12+cos2B+12+cos2C+12
=12(cos2A+cos2B+cos2C+3)
=12(2cos(A+B)cos(A−B)+cos(π−2(A+B))+3)
=12(2cos(A+B)cos(A−B)−cos2(A+B)+3)
=12(2cos(A+B)cos(A−B)−cos2(A+B)+3)
=12(2cos(A+B)cos(A−B)−2cos2(A+B)+4)
=12(2cos(A+B)(cos(A−B)−cos(A+B))+4)
=2+2sinAsinBsinC