>>772
y = (tan(x))^(cos(x))
log(y) = cos(x) log(tan(x))
(d/dx) log(tan(x)) = (1+tan(x)^2)/(tan(x))
(d/dx) {1/cos(x)} = sin(x)/(cos(x)^2) = tan(x)/cos(x)

ロピタルから
log(y) = cos(x) log(tan(x)) → 0
y → 1