lim(x->+∞) [√(4x^6-5x^5) -2x^3]/(27x^6+8x)^(1/3)
分子分母同时乘以 [√(4x^6-5x^5) +2x^3]
=lim(x->+∞) [(4x^6-5x^5) -4x^6]/{ (27x^6+8x)^(1/3).[√(4x^6-5x^5) +2x^3] }
=lim(x->+∞) -5x^5/{ (27x^6+8x)^(1/3).[√(4x^6-5x^5) +2x^3] }
分子分母同时除x^5
=lim(x->+∞) -5/{ (27+8/x^5)^(1/3).[√(4-5/x) +2] }
=-5/[(3)(4)]
=-5/12