Làm tính nhân:

a) 3x.(5x² − 2x − 1);

b) (x² + 2xy − 3).(− xy);

c) \(\frac{1}{2}{x^2}y\,.\,\left( {2{x^3} - \frac{2}{5}x{y^2} - 1} \right)\).

Lời giải

a) 3x.(5x² − 2x − 1)

= 3x.5x² − 3x.2x − 3x.1

= 15x³ − 6x² − 3x 

b) (x² + 2xy − 3).(– xy)

= x².(− xy) + 2xy.(− xy) − 3.(− xy)

= − x³y − 2x²y² + 3xy

c) \(\frac{1}{2}{x^2}y\,.\,\left( {2{x^3} - \frac{2}{5}x{y^2} - 1} \right)\)

\( = \frac{1}{2}{x^2}y\,.\,2{x^3} - \frac{1}{2}{x^2}y\,.\,\frac{2}{5}x{y^2} - \frac{1}{2}{x^2}y\,.\,1\)

\( = {x^5}y - \frac{1}{5}{x^3}{y^3} - \frac{1}{2}{x^2}y\)