求极限 lim（cosx—cos3x)/x^2 其中x趋向于0.我只算出来是2,

cos3x = cosx cos2x - sinx sin2x
cosx - cos3x = cosx ( 1- cos2x) + sinx sin2x = cosx * 2 sin²x + sinx sin2x
lim(x->0) (cosx - cos3x) / x²
= lim(x->0) [ cosx * 2 sin²x + sinx sin2x ] / x²
= 2 + 2 = 4