a) x(x-y) + y(x-y)
= (x - y)(x + y)
= x² - y²
b) $x^{n-1}$ (x+y) - y ( $x^{n-1}$ + $y^{n-1}$)
= x . $x^{n-1}$ + y . $x^{n-1}$ - y . $x^{n-1}$ - y . $y^{n-1}$
= x . $x^{n-1}$ - y . $y^{n-1}$
= $x^{n-1+1}$ - $y^{n-1+1}$
= $x^{n}$ - $y^{n}$
~ CHÚC BẠN HỌC TỐT ~