Đáp án:
a, x² + 2x + 1= (x+1)²
b,9x² + y² + 6xy = (3x)² + 2 . 3xy +y² =(3x+y)²
c,25a²-20ab + 4b² = (5a)² - 2.5a.2b + (2b)²=(5a-2b)²
d,x² - x + $\frac{1}{4}$ = (x-$\frac{1}{2}$ )²
e, $a^{4}$ + 4b² +4a²b =(a²)²+2.2a²b + (2b)² = (a²+2b)²
f, 4a²b² +9 -12ab = (2ab)² - 2.2ab.3 +3² = (2ab-3)²