Giải toán
a) \((2x^2 + 3y)^3\)
\(= (2x^2)^3 + 3.(2x^2)^2.3y + 3.2x^2.(3y)^2 + (3y)^3\)
\(= 8x^6 + 36x^4y + 54x^2y^2 + 27y^3\)
b) \(\left(\dfrac{1}{2}x -3\right)^3\)
\(= \left(\dfrac{1}{2}x\right)^3 - 3. \left(\dfrac{1}{2}x\right)^2.3 + 3. \dfrac{1}{2}x.3^2 - 3^3\)
\(=\dfrac{1}{8}x^3 - \dfrac{9}{4}x^2 + \dfrac{27}{2}x - 27\)
Lưu ý:
\((a + b)^3 = a^3 + 3a^2b + 3ab^2 + b^3\)
\((a - b)^3 = a^3 - 3a^2b + 3ab^2 - b^3\)
