Bài 1 trang 5 sgk Toán 8 tập 1: Làm tính nhân:...

1. Làm tính nhân:

a) x²(5x³ – x - $\frac{1}{2}$);

b) (3xy – x² + y) $\frac{2}{3}$x²y;

c) (4x³– 5xy + 2x)(- $\frac{1}{2}$xy).

a) x²(5x³ – x - $\frac{1}{2}$) = x². 5x³ + x² . (-x) + x² . (-$\frac{1}{2}$)

                           = 5x⁵ – x³ – $\frac{1}{2}$x²

b) (3xy – x² + y) $\frac{2}{3}$x²y = $\frac{2}{3}$x²y . 3xy + $\frac{2}{3}$x²y . (- x²) + $\frac{2}{3}$x²y . y

                                  = 2x³y² – $\frac{2}{3}$x⁴y + $\frac{2}{3}$x²y²

c) (4x³– 5xy + 2x)(- $\frac{1}{2}$xy) = - $\frac{1}{2}$xy . 4x³ + (- $\frac{1}{2}$xy) . (-5xy) + (- $\frac{1}{2}$xy) . 2x

                                      = -2x⁴y + $\frac{5}{2}$x²y² - x²y.