摘要：21．(1)(a2b)3÷(ab2)2×a3b2, (2)(+3y)2-(-3y)2, (3)(2a-3b+1)2, (4)(x2-2x-1)(x2+2x-1), (5)(a-b)(2a+b)(3a2+b2), (6)[(a-b)(a+b)]2÷(a2-2ab+b2)-2ab．

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