>>78
y={cos(a/x)}^(x^2) とおいて対数をとると、log(y)=(x^2)*log{cos(a/x)}、右辺について
lim[x→∞] (x^2)*log{cos(a/x)} = lim[x→∞] log{cos(a/x)}/(1/x^2) は、不定形になるから
ロピタル1回で、lim[x→∞] log{cos(a/x)}/(1/x^2) = (-a/2)lim[x→∞] x*tan(a/x)
=(-a/2)lim[x→∞] x*tan(a/x)=(-a^2/2)lim[x→∞] tan(a/x)/(a/x) = (-a^2/2)*1 = -a^2/2
よって、lim[x→∞] log(y) = -a^2/2　⇔　lim[x→∞] y = e^(-a^2/2)