\[\left( \frac{1}{b^{- 3}} + \frac{1}{c^{- 3}} \right) \cdot (b + c)^{- 1} =\]
\[= \ \frac{\left( b^{3} + c^{3} \right)}{b + c} =\]
\[= b^{2} - bc + c^{2}\]