Làm tính nhân:

$$\begin{gathered} a) (2x^2 – 3x\left)\right(5x^2 – 2x + 1) \\ b) (x – 2y)(3xy + 5y^2 + x) \end{gathered}$$

Giải:

$$\begin{gathered} a) (2x^2 – 3x\left)\right(5x^2 – 2x + 1) \\ = 2x^2.5x^2 - 2x^2.2x + 2x^2–3x.5x^2 +3x.2x-3x \\ = 10x^4 – 4x^3 + 2x^2 – 15x^3 + 6x^2 – 3x \\ = 10x^4 – 19x^3 + 8x^2 – 3x \end{gathered}$$

$$\begin{gathered} b) (x – 2y\left)\right(3xy + 5y^2 + x) \\ =x.3xy+x.5y^2+x.x-2y.3xy-2y.5y^2-2y.x \\ = 3x^{2}y + 5x{y}^2 + x^2 – 6x{y}^2 – 10y^3 – 2xy \\ = 3x^{2}y – x{y}^2 – 2xy+ x^2 – 10y^3 \end{gathered}$$