sinπ/12*sin5π/12
=-1/2[cos(π/12+5π/12)-cos(π/12-5π/12)]
=-1/2[cos(π/2)-cos(-π/3)]
=-1/2[0-1/2]
=1/4
公式:
sinαsinβ = [cos(α-β)-cos(α+β)] /2
cosαcosβ = [cos(α+β)+cos(α-β)]/2
sinαcosβ = [sin(α+β)+sin(α-β)]/2
cosαsinβ = [sin(α+β)-sin(α-β)]/2