theorem Th6: :: FIB_NUM:6

for x, a, b, c being Real st a <> 0 & delta (a,b,c) >= 0 holds
( ((a * (x ^2)) + (b * x)) + c = 0 iff ( x = ((- b) - (sqrt (delta (a,b,c)))) / (2 * a) or x = ((- b) + (sqrt (delta (a,b,c)))) / (2 * a) ) )