Giải thích các bước giải:
4)
a) $3x^{3}.\left ( x^{2} - 5x + 1 \right )$
$= 3x^{3}.x^{2} - 3x^{3}.5x + 3x^{3}.1$
$= 3x^{5} - 15x^{4} + 3x^{3}$
b) $\left ( -2x \right ).\left ( x^{2} + 2x - 4 \right )$
$= \left ( -2x \right ).x^{2} + \left ( -2x \right ).2x + \left ( -2x \right ).\left ( -4 \right )$
$= -2x^{3} - 4x^{2} + 8x$
c) $\left ( x + 1 \right ).\left ( x - 6 \right )$
$= x\left ( x - 6 \right ) + 1.\left ( x - 6 \right )$
$= x^{2} - 6x + x - 6$
$= x^{2} - 5x - 6$
d) $\left ( x - 2 \right ).\left ( x + 8 \right )$
$= x\left ( x + 8 \right ) - 2\left ( x + 8 \right )$
$= x^{2} + 8x - 2x - 16$
$= x^{2} + 6x - 16$
e) $\left ( x + 2 \right ).\left ( x^{2} - 5x + 7 \right )$
$= x\left ( x^{2} - 5x + 7 \right ) + 2\left ( x^{2} - 5x + 7 \right )$
$= x^{3} - 5x^{2} + 7x + 2x^{2} - 10x + 14$
$= x^{3} - 3x^{2} - 3x + 14$
f) $\left ( x - 3 \right ).\left ( x^{2} + 4x - 9 \right )$
$= x\left ( x^{2} + 4x - 9 \right ) - 3\left ( x^{2} + 4x - 9 \right )$
$= x^{3} + 4x^{2} - 9x - 3x^{2} - 12x + 27$
$= x^{3} + x^{2} - 21x + 27$
5) $2x\left ( 5 - x \right ) - x\left ( 3 + 2x \right ) = 26$
$\Leftrightarrow 10x - 2x^{2} - 3x - 2x^{2} = 26$
$\Leftrightarrow -4x^{2} + 7x = 26$
$\Leftrightarrow 4x^{2} - 7x + 26 = 0 (VN)$
