a x 2 + b x + c = 0 , a 0
D = b 2 - 4 a c
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a x 2 + b x + c = a [ x 2 - ( x 1 + x 2 ) x + x 1 . x 2 ] ili a x 2 + b x + c = a ( x - x 1) ( x - x 2)