(3^X+9^X)^1/x
=[3^X(1+3^X)]^(1/X)
=3(1+3^X)^(1/X)
lim(x->∞)3(1+3^X)^(1/X)
=3 lim(x->∞) (3^x)^(1/x)
=3×3
=9