分子有理化：
=lim[√(x+1)+2][x-3]/[x-3][(x+5)^(2/3)+2(x+5)^(1/3)+4]
=lim[√(x+1)+2]/(x+5)^(2/3)+2(x+5)^(1/3)+4]
=(2+2)/(4+4+4)
=1/3