b1
x²y (2x - 2y)
= (x²y. 2x) + (x²y. -2y)
= 2x³y -2x²y²
5x (2x + 4x²y)
= (5x. 2x) + (5x. 4x²y)
= 10x² + 20x³y
b2
(x-y).(x+y)
=(x.x) + (x.y) +(-y.x) + (-y.y)
= x² + xy- xy -y²
= x² - y²
(x-1) . (x+1) . (x+2)
= [(x.x)+ (x.1) +(-1.x) + (-1.1)] . (x+2)
=(x² +x -x+1) (x+2)
=(x²+1) .(x+2)
=(x².x) + (x².2) + (1.x) + (1.2)
=x³ + 2x² + x + 2
bài 3 chưa học :))