b) (x^2-1/3xy+1/9y^2)(x+1/3y)

b) $\left(x^2-\frac{1}{3}xy+\frac{1}{9}{y}^2\right)\left(x+\frac{1}{3}y\right).$

Trả lời

b)  $\left(x^2-\frac{1}{3}xy+\frac{1}{9}{y}^2\right)\left(x+\frac{1}{3}y\right)$

 $=x^2\cdot \left(x+\frac{1}{3}y\right)-\frac{1}{3}xy\cdot \left(x+\frac{1}{3}y\right)+\frac{1}{9}{y}^2\cdot \left(x+\frac{1}{3}y\right)$

$=x^3+\frac{1}{3}{x}^{2}y-\frac{1}{3}{x}^{2}y-\frac{1}{9}x{y}^2+\frac{1}{9}x{y}^2+\frac{1}{27}{y}^3$

$=x^3+\left(\frac{1}{3}{x}^{2}y-\frac{1}{3}{x}^{2}y\right)+\left(-\frac{1}{9}x{y}^2+\frac{1}{9}x{y}^2\right)+\frac{1}{27}{y}^3$

$=x^3+\frac{1}{27}{y}^3$.