sina×sinb
=sina*sin(2π/3-a)
=sina*(√ 3/2cosa+1/2sina)
=1/2(sina)^2+√ 3/2sinacosa
=1/4+1/2(√ 3/2sin2a-1/2cos2a)
=1/4-1/2cos(2a+π/3)
π/3
sina×sinb
=sina*sin(2π/3-a)
=sina*(√ 3/2cosa+1/2sina)
=1/2(sina)^2+√ 3/2sinacosa
=1/4+1/2(√ 3/2sin2a-1/2cos2a)
=1/4-1/2cos(2a+π/3)
π/3