利用1-cost的等价无穷小为t^2/2来做 (t趋于0)
分子化为coa2x-1+1-cos3x
(coa2x-1+1-cos3x)/√(1+x^2)-1
=(coa2x-1+1-cos3x)*(√(1+x^2)+1)/(√(1+x^2)-1)*(√(1+x^2)-1)
=(coa2x-1+1-cos3x)*(√(1+x^2)+1)/(1+x^2-1)
=(coa2x-1+1-cos3x)*(√(1+x^2)+1)/x^2
=(coa2x-1)*(√(1+x^2)+1)/x^2+(1-cos3x)*(√(1+x^2)+1)/x^2
=(-(2x)^2/2)*(√(1+x^2)+1)/x^2+((3x)^2/2)*(√(1+x^2)+1)/x^2
=-4+9
=5