x趋向于0 (tan3x-tanx)/x
问题描述：
x趋向于0 (tan3x-tanx)/x
答
lim[x-->0](tan3x-tanx)/x
=lim[x-->0]tan3x/x-lim[x-->0]tanx/x
=lim[x-->0]sin3x/(xcos3x)-lim[x-->0]sinx/(xcosx)
=3-1
=2