1 (a+3)(a-3)
2 -(x-y)²
3 (a+3)(a-4)
4 a^2+b^2-4a^2b^2
=a^2+b^2-4a^2b^2+2ab-2ab
=(a+b)^2-4a^2b^2-2ab
=(a+b)^2-4a^2b^2-2ab+1/4-1/4
=(a+b)^2-(2ab+1/2)^2+1/4
=(a+b+2ab+1/2)(a+b-2ab-1/2)+1/4
5 (x+y+1)(x-y)
6 (3a-3b+1)²
祝你好好学习，天天向上
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