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distributive property:
v(v^2-vf+f^2) + f(v^2-vf+f^2)
= v^3 - v^2 f + v f^2
_____ + f v^2 - v f^2 + f^3
----------------------------- add
= v^3 + 0 - 0 + f^3
= v^3 + f^3