નિત્યસમ $(vi)$ : $(x-y)^3=x^3-y^3-3 x y(x-y)$
$\left(x-\frac{2}{30} y\right)^{3=(x)^3-}\left(\frac{2}{3} y \right)^{3-3(x)}\left(\frac{2}{3} y \right)\left( x -\frac{2}{3} y \right)$
$= x ^3-\frac{8}{27} y ^3-2 xy \left( x -\frac{2}{3} y \right)$
$= x ^3-\frac{28}{7} y ^3-\left[(2 xy ) x -(2 xy ) \frac{2}{3} y \right]$
$= x ^3-\frac{8}{27} y ^3-\left[2 x ^2 y -\frac{4}{3} xy ^2\right]$
$= x ^3-\frac{8}{27} y ^3-2 x ^2 y -\frac{4}{3} xy ^2$
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