lim(x→0) [cos(3x)－cos(5x)] / x^2
＝lim(x→0) [2×sin(4x)×sinx] / x^2
＝lim(x→0) [2×4x×x] / x^2
＝8
利用：x→0时,sinx与x是等价无穷小,所以sin(4x)与4x等价