arcsinx=x+1/6x^3+o(x^3)
tanx=x+1/3x^3+o(x^3)
arctanx=x-1/3x^3+o(x^3)
cosx=1-1/2x^2+1/24x^4
ln(1+x)=x-1/2x^2+1/3x^3+o(x^3)
e^x=1+x+1/2x^2+1/3x^3+...+o(x^3)
(1+x)^2=1+2x+a(a-1)^2x^2/2!