1-cosxcos2xcos3x=1-cos3x++cos3x(1-cos2x)+cos2xcos3x(1-cosx)~(3x)^2/2+(2x)^2/2+x^2/2=7x^2
(等价无穷小)
1-cosx~x^2/2
原式=lim{x->0}7x^2/(x^2/2)=14