lim(x->无穷) ln(1+3x^2)/ln(3+x^4)洛必达=lim(x->无穷) [6x/(1+3x^2)]/[4x^3/(3+x^4)]=lim(x->无穷) [3x(3+x^4)]/[2x^3.(1+3x^2)]分子分母同时除x^5=lim(x->无穷) [3(3/x^4+ 1)]/[2(1/x^2+3)]=3(0+1)/[2(0+3)]=1/2
