given that a + b + 1 = 0
we know if a+b+c = 0
then $$a^3 + b^3 + c^3 - 3abc$$ = 0
as we can see that $$(a^3 + b^3 +1 - 3ab)$$ resembles $$a^3 + b^3 + c^3 - 3abc$$ with c = 1
and hence $$(a^3 + b^3 +1^3 - (3ab \times 1))$$ = 0
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