Bài 3 Tính:

$a$) $\left(2y-1\right)^3=2y^3-3.\left(2y\right)^2.1+3.2y.1^2-1^3$

$=2y^3-12y^2+6y-1$

b)$\left(3x^2+2y\right)^3$

$=\left(3x^2\right)^3=3.\left(3x^2\right)^2.2y+3.\left(3x^2\right).\left(2y\right)^2+\left(2y\right)^3$

$=27x^8+3.9x^4.2+9x^2.4y+8y^3$

$=27x^8+54x^4+36x^2y+8y^3$

c)$\left(\dfrac{1}{3}x-2\right)^3$

$=\left(\dfrac{1}{3}x\right)^3-3.\left(\dfrac{1}{3}x\right)^2.2+3.\dfrac{1}{3}x.2^2-2^3$

$=\dfrac{1}{27}x^3-3.\dfrac{1}{9}x^2.2+x.2^2-8$

$=\dfrac{1}{27}x^3-\dfrac{2}{3}x^2+4x-8$