e^u=1+1/2u^2+1/6u^3+o(u^3)
sinx=x-1/3x^3+o(x^3)
e^sinx=1+sinx+1/2sinx^2+1/6sinx^3+o(sinx^3)
=1+[x-1/3x^3+o(x^3)]+1/2[x-1/3x^3+o(x^3)]^2+1/6[x-1/3x^3+o(x^3)]^3+o(x-1/3x^3+o(x^3))
将[x-1/3x^3+o(x^3)]^2,[x-1/3x^3+o(x^3)]^3,展开时,超过x^3的归到o(x^3)
故写成了1/2[x+o(x^3)]^2,1/6[x+o(x^3)]^3+o(x^3)