a) $2x^2(5x^3-4x^{2}y-7xy+1)$
$\iff 2x^2.5x^3-2x^2.4x^2.y+2x^2.(-7xy)+2x^2.1$
$\iff 10x^5-8x^{4}y-14x^{3}y+2x^2$
b) $2x(3x^3-x)-4x^2(x-x^2+1)+(x-3x^2)x$
$\iff 2x.3x^3+2x.(-x)-[4x^2.x+4x^2.(-x^{2 })+4x^2.1]+x.x-3x^2.x$
$\iff 6x^4-2x^2-(4x^3-4x^4+4x^2)+x^2-3x^3$
$\iff 6x^4-2x^2-4x^3+4x^4-4x^2+x^2-3x^3$
$\iff (6x^4-2x^2)-(4x^3+3x^3)-(2x^2+4x^2-x^2)$
$\iff 10x^4-7x^3-5x^2$

đây chị nha :)

Làm tính Nhân 
2x^2(5x^3 − 4x^2y − 7xy + 1)
= 2x^2.5x^3 − 2x^2.4x^2y − 2x^2.7xy + 2x^2.1
= 10x^5 - 8x^4y - 14x^3y + 2x^2
Rút gọn biểu thức: 
2x(3x^3 − x) − 4x^2(x − x^2 + 1) + (x − 3x^2)x
= 6x^4 - 2x^2 - 4x^3 + 4x^4 - 4x^2 + x^2 - 3x^3
= 10x^4 - 7x^3 - 5x^2x2(5x3−4x2y−7xyx2