也可以用泰勒展开泰勒具体怎么展开啊，消不掉的样子啊sinx=x-x^3/3!+o(x^3)x^(1/2)-(sinx)^(1/2)=(x-sinx)/[x^(1/2)+(sinx)^(1/2)]=[x^3/3!-o(x^3)]/[x^(1/2)+(sinx)^(1/2)]=[x^2.5/3!-o(x^2.5)]/[1+(sinx/x)^(1/2)]分母趋向2，分子为x^2.5/3!，所以等价无穷小x^2.5/12