这个是极限的等价公式,来源于泰勒公式,需要对极限的理论有了解才能理解
x->0
cosx = 1 -(1/2)x^2 +o(x^2)
cos2x =1 -(1/2)(2x)^2 +o(x^2) = 1-2x^2 +o(x^2)
cosx.cos2x
=[1 -(1/2)x^2 +o(x^2)].[1 -2x^2 +o(x^2)]
=1 -(2+1/2)x^2 +o(x^2)
=1- (5/2)x^2 +o(x^2)
cos3x =1 -(1/2)(3x)^2 +o(x^2) = 1- (9/2)x^2 +o(x^2)
cosx.cos2x.cos3x
=[1- (5/2)x^2 +o(x^2)] .[ 1- (9/2)x^2 +o(x^2)]
=1 - (5/2 +9/2)x^2 +o(x^2)
=1-7x^2+o(x^2)
1-cosx.cos2x.cos3x =7x^2+o(x^2)
lim(x->0) [1-cosx.cos2x.cos3x] / (ax^n) =1
lim(x->0) 7x^2 / (ax^n) =1
a=7 , n=2