Mn làm giúp em 3 bài này với ạ. E cảm ơn

Bài $1$:

$a)$ $\left(-\frac{1}{7}x^2yz\right)(-by)\left(-\frac{2}{5}xz^3\right)$

$= \left[\left(-\frac{1}{7}\right) \cdot (-b) \cdot \left(-\frac{2}{5}\right)\right] \cdot (x^2 \cdot x) \cdot (y \cdot y) \cdot (z \cdot z^3)$

$= -\frac{2b}{35}x^3y^2z^4$

$b)$ $\left(-\frac{7}{2}xy\right)\left(-\frac{4}{7}x^2\right)\left(-\frac{1}{2}xy\right)^3$

$= \left(-\frac{7}{2}xy\right) \cdot \left(-\frac{4}{7}x^2\right) \cdot \left(-\frac{1}{8}x^3y^3\right)$

$=\left[\left(-\frac{7}{2}\right) \cdot \left(-\frac{4}{7}\right) \cdot \left(-\frac{1}{8}\right)\right] \cdot (x \cdot x^2 \cdot x^3) \cdot (y \cdot y^3)$

$= -\frac{1}{4}x^6y^4$

Bài $2$:

$a)$ $-\frac{1}{2}xy^2z + 3x^3y^2 + 2xy^2z - \frac{2}{3}xy^2z - \frac{1}{3}x^3y^2 + xy^2z$

$= \left(3x^3y^2 - \frac{1}{3}x^3y^2\right) + \left(-\frac{1}{2}xy^2z + 2xy^2z - \frac{2}{3}xy^2z + xy^2z\right)$

$= \frac{8}{3}x^3y^2 + \left(-\frac{1}{2} + 2 - \frac{2}{3} + 1\right)xy^2z$

$= \frac{8}{3}x^3y^2 + \frac{11}{6}xy^2z$

Bài $3$:

$a)$ $x^4y^5 \left(\frac{7}{3}x^2y^4 - \frac{1}{7}x^3y - \frac{5}{14}\right)$

$= x^4y^5 \cdot \frac{7}{3}x^2y^4 - x^4y^5 \cdot \frac{1}{7}x^3y - x^4y^5 \cdot \frac{5}{14}$

$= \frac{7}{3}x^6y^9 - \frac{1}{7}x^7y^6 - \frac{5}{14}x^4y^5$

$b)$ $3x^4 - 4x^3 + 2x(x^3 - 2x^2 + 7x)$

$= 3x^4 - 4x^3 + 2x \cdot x^3 - 2x \cdot 2x^2 + 2x \cdot 7x$

$= 3x^4 - 4x^3 + 2x^4 - 4x^3 + 14x^2$

$= (3x^4 + 2x^4) + (-4x^3 - 4x^3) + 14x^2$

$= 5x^4 - 8x^3 + 14x^2$

$c)$ $(3x + 5)(2x - 7)$

$= 3x \cdot 2x - 3x \cdot 7 + 5 \cdot 2x - 5 \cdot 7$

$= 6x^2 - 21x + 10x - 35$

$= 6x^2 - 11x - 35$

$d)$ $(m^2 - 3m)(2m^3 - 3m^2)$

$= m^2 \cdot 2m^3 - m^2 \cdot 3m^2 - 3m \cdot 2m^3 + 3m \cdot 3m^2$

$= 2m^5 - 3m^4 - 6m^4 + 9m^3$

$= 2m^5 - 9m^4 + 9m^3$

$\text{$\color{#E8BC1C}{T}\color{#E8BC1C}{o}\color{#E8BC1C}{a}\color{#E8BC1C}{n}\color{#5396B2}{H}\color{#5396B2}{o}\color{#5396B2}{c}\color{#404040}{H}\color{#404040}{D}$}$