sin75度=sin(45+30)=（根号6+根号2）/4
sin165=sin15=sin(45-30)=（根号6-根号2）/4
原式=sin75sin(180-15)
=sin75sin15
=sub75cos(90-15)
=sin75cos75
=1/2*(2sin75cos75)
=1/2*sin150
=1/4
sin75sin165
＝sin75sin15
= (cos(75度-15度)-cos(75度+15度))/2
= (1/2-0)/2
= 1/4
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