原式=sin 15° cos 15° [ (cos 15°)^2 -(sin 15°)^2 ]
=(1/2) *(2 sin 15° cos 15°) *cos 30°
=(1/2) *sin 30° *(根号3)/2
=(1/2) *(1/2) *(根号3)/2
= (根号3)/8.
= = = = = = = = =
分解因式,再用二倍角公式.
已用计算器验证.