a²-4ab+4b²
=a²-2×a×2b+(2b)²
=(a-2b)²
4+12（x-y）+9（x-y）²
=2²+2×2×3(x-y)+[3(x-y)]²
=[2+3(x-y)]²
=(2+3x-3y)²
分解因式：a²-4ab+4b²；4+12（x-y）+9（x-y）²
a²-4ab+4b²
=a²-2×a×2b+(2b)²
=(a-2b)²
4+12（x-y）+9（x-y）²
=2²+2×2×3(x-y)+[3(x-y)]²
=[2+3(x-y)]²
=(2+3x-3y)²