Đáp án:
a, ( x+y+4)(x+y-4)
=[(x+y) + 4].[(x+y) - 4]
=(x+y)² - 4²
=(x+y)² - 16
b, x² -2x +1 -2(x-1)y+y²
= (x-1)² -2(x-1)y+y²
=[(x-1) - y]²
=(x-1-y)²
c, 9x² + 2x + $\frac{1}{9}$
= (3x)² + 2.3x.$\frac{1}{3}$ + ($\frac{1}{3}$ )$^{2}$
=(3x + $\frac{1}{3}$ )$^{2}$
h, x² + 2xy + y² + 4x + 4y + 4
=(x² + 2xy +y²) +4(x + y)+4
=(x + y)² + 2.2.(x+y)+2²
=(x+y+2)²