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这个题要用洛必达法则，上下求导，需要知道cos3x等价无穷小是多少，x^3求导是3星^2，再看题目是趋向于∞还是0，再进行下一步计算
cos3x除以x三次方的极限
(x→0）lim{(1-cosx)/x³}
= (x→0）lim{[1-1+2sin²(x/2)]/x³}
= (x→0）lim{2sin²(x/2)]/8*(x/2)³}
= (x→0）lim{sin²(x/2)]/4*(x/2)³}
= (x→0）lim{(x/2)²]/4*(x/2)³}
= +∞
(x→0）lim{(1-cosx)/x³}
= (x→0）lim{[1-1+2sin²(x/2)]/x³}
= (x→0）lim{2sin²(x/2)]/8*(x/2)³}
= (x→0）lim{sin²(x/2)]/4*(x/2)³}
= (x→0）lim{(x/2)²]/4*(x/2)³}
= +∞
(x→0）lim{(1-cosx)/x³}
= (x→0）lim{[1-1+2sin²(x/2)]/x³}
= (x→0）lim{2sin²(x/2)]/8*(x/2)³}
= (x→0）lim{sin²(x/2)]/4*(x/2)³}
= (x→0）lim{(x/2)²]/4*(x/2)³}
= +∞