Question 639861
a^-1 - b^-1
__________
a^-3 - b^-3

=(1/a-1/b)/(1/a^3-1/b^3)
=[(b-a)/ab]/[(b^3-a^3)/a^3b^3]
=[(b-a)/(b^3-a^3)]*[a^3b^3/ab]
=[(b-a)/(b-a)(b^2+a^2+ab)]*[a^3b^3/ab] 
=[1/(b^2+a^2+ab)]*[a^2b^2]
=a^2b^2/(b^2+a^2+ab) ans