Question
Expand $(5k + l)^3 $
Answer$\;\;\;(5k + l)^3 $
$=(A + B)^3 \text{ , where } A = 5k \text{ and } B= l$
$=A^3 + 3A^2B + 3AB^2 + B^3 $
$=(5k)^3 + 3(5k)^2(l) + 3(5k)(l)^2 + (l)^3 $
$= 125k^{3} + 3(25k^{2})(l) + 3(5k)(l^{2}) + l^{3} $
$= 125k^{3} + 75k^{2}l + 15kl^{2} + l^{3} $