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Який многочлен треба додати до тричлена 2a2 – 5a + 7, щоб сума дорівнювала: 1) (2a2 – 5a + 7) + (–2a2 + 5a – 2) = 2a2 – 5a + 7 – 2a2 + 5a – 2 = 5; 2) (2a2 – 5a + 7) + (–2a2 + 5a – 2) = 2a2 – 5a + 7 – 2a2 + 5a – 7 = 0; 3) (2a2 – 5a + 7) + (–a2 + 5a – 7) = 2a2 – 5a + 7 – a2 + 5a – 7 = a2; 4) (2a2 – 5a + 7) + (–2a2 + 3a – 7) = 2a2 – 5a + 7 – 2a2 + 3a – 7 = – 2a.