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a)x²+2x+1=0
(x+1)²=0 x1,2=-1
c) a≠0
d)x²-9=0
(x-3)(x+3)=0. x1=3. x2=-3
e)2x²+3x+1=0
2x²+2x+x+1=0
2x(x+1)+x+1=0
(x+1)(2x+1)=0
f)x²=-1 fals