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a) 12x-y+2 ⇒ irréductible 
b) 7y+12-13y = -6y+12
c) 10-8d+3 = -8d+13
d) 8-x+x²+5x = x²+4x+8
e) 3t-12t+t²-7 = t²-9t-7
f) a²+b-a+3b = a²+4b-a