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12. Efectuați calculele: a) (2-3x)²+(2+ 3x)2-(2x-3)²-(4x-3)(4x + 3); b) (2x² + y)²-(2x²-y)² + (x²+y)(x²-y); d) (2x + 5)²-(3x - 5)² + (2x-7)(2x + 7); 13. Efectuați calculele: c) (x + 3)² + (x + 1)²-(2x + 1)(2x - 1); c) (x+4)²-(2x-3)(2x + 3) + (2x + 1)². ​