Phân tích đa tích thành nhân tử

$a) 3(x-y)-5x(y-x)$
$=3(x-y)+5x(x-y)$
$= (x-y)(3+5x)$.
$b)$ $\frac{2}{5}x(y-1)+$ $\frac{2}{5}y(1-y)$

$=$ $\frac{2}{5}x(y-1)-$ $\frac{2}{5}y(y-1)$

$=$ $(y-1)$ . $(\frac{2}{5}x - \frac{2}{5}y)$

$=$ $(y-1)$ . $\frac{2}{5}(x-y).$