Special Products Rule 6

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Expand $(9k + l)(81k^{2} - 9kl +l^{2})$

**Answer**

$\;\;\;(9k + l)(81k^{2} - 9kl +l^{2})$

$=(9k + l)((9k)^2 - (9k)(l) + (l)^2) $

$=(A + B)(A^2 - AB + B^2) \text{ , where } A = 9k \text{ and } B = l $

$=A^3 + B^3 $

$=(9k)^3 + (l)^3 $

$=729k^{3} + l^{3} $