If $a - b = 3$ and $ab = 10$, find $a^{3} - b^{3}$.

If $a - b = 3$ and $ab = 10$, find $a^{3} - b^{3}$.
  1. $117$
  2. $90$
  3. $57$
  4. $27$

Solution

$a^{3}-b^{3} = (a-b)^{3} + 3ab(a-b) = 27 + 3 \cdot 10 \cdot 3 = 27+90 = 117$.

Asked in: IMO

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