(b/a)^2÷（a/b)-b^3/a^3(a-b) ÷ 1/(a-b )
=(b/a)^2×（b/a)-b^3×(a-b )/a^3(a-b)
=(b/a)^3-b^3/a^3
=(b/a)^3-(b/a）^3
＝0