Bài 7.24 trang 38 toán lớp 7 tập 2 KNTTRút gọn các biểu thức sau:a) $4x^2(5x^2 + 3) – 6x(3x^3 – 2x...

a)
$4x^2(5x^2 + 3) – 6x(3x^3 – 2x + 1) – 5x^3(2x – 1)$
$= 4x^2. 5x^2 + 4x^2 . 3 + (-6x). 3x^3 + (-6x).(-2x) + (-6x).1 + (-5x^3).2x + (-5x^3). (-1)$
$= 20x^4 + 12x^2 – 18x^4 + 12x^2 – 6x – 10x^4 + 5x^3$
$= -8x^4 + 5x^3 + 24x^2 – 6x$

b)
$\frac{3}{2}x(x^2 - \frac{2}{3}x + 2) - \frac{5}{3}x^2(x + \frac{6}{5})$
$= \frac{3}{2}x . x^2 + \frac{3}{2}x . (- \frac{2}{3}x) + \frac{3}{2}x . 2 + (-\frac{5}{3}x^2). x + (-\frac{5}{3}x^2) . \frac{6}{5})$
$= \frac{3}{2}x^3 – x^2 + 3x -\frac{5}{3}x^3 - 2x^2$
$= -\frac{1}{6}x^3 – 3x^2 + 3x$