sin15度乘sin75度乘cos30度
=sin15度乘cos15度乘cos30度=2分之1(sin30度)乘cos30度=4分之1(sin60度)
=8分之根号3 cos43cos77+cos313cos167=cos43cos77+cos(360-47)cos(90+77)=cos43cos77-cos47sin77=cos43cos77-sin43sin77=cos(43+77)=cos120=-cos30=负2分之根号3
sin15度乘sin75度乘cos30度
=sin15度乘cos15度乘cos30度=2分之1(sin30度)乘cos30度=4分之1(sin60度)
=8分之根号3 cos43cos77+cos313cos167=cos43cos77+cos(360-47)cos(90+77)=cos43cos77-cos47sin77=cos43cos77-sin43sin77=cos(43+77)=cos120=-cos30=负2分之根号3