>>527
Tan(X)=x+(1/3)x^3+(2/15)x^5+O(x^6)
ArcTan(X)=x-(1/3)x^3+(1/5)x^5+O(x^6)
より
Tan(X)*ArcTan(X)-X^2=(2/9)X^6+O(X^7)
従って
lim (Tan(X)*ArcTan(X)-X^2)/X^6＝2/9
X→0