<Dd> a (− b 2 a + q) 2 + b (− b 2 a + q) + c = 0 (− b 2 a + q) 2 + b a (− b 2 a + q) + c a = 0 (a ≠ 0) b 2 4 a 2 + q 2 − b q a − b 2 2 a 2 + b q a + c a = 0 − b 2 4 a 2 + q 2 + c a = 0 q 2 = b 2 − 4 a c 4 a 2 q = ± b 2 − 4 a c 2 a (\ displaystyle (\ begin (aligned) a \ left ((\ frac (- b) (2a)) + q \ right) ^ (2) + b \ left ((\ frac (- b) (2a)) + q \ right) + c& = 0 \ \ (5pt) \ left ((\ frac (- b) (2a)) + q \ right) ^ (2) + (\ frac (b) (a)) \ left ((\ frac (- b) (2a)) + q \ right) + (\ frac (c) (a)) & = 0&& (a \ neq 0) \ \ (5pt) (\ frac (b ^ (2)) (4a ^ (2))) + q ^ (2) - (\ frac (bq) (a)) - (\ frac (b ^ (2)) (2a ^ (2))) + (\ frac (bq) (a)) + (\ frac (c) (a)) & = 0 \ \ (5pt) (\ frac (- b ^ (2)) (4a ^ (2))) + q ^ (2) + (\ frac (c) (a)) & = 0 \ \ (5pt) q ^ (2) & = (\ frac (b ^ (2) - 4ac) (4a ^ (2))) \ \ (5pt) q& = (\ frac (\ pm (\ sqrt (b ^ (2) - 4ac))) (2a)) \ \ (5pt) \ end (aligned))) </Dd> <P> The value of x in the extreme point is then added to both sides of the equation </P> <Dl> <Dd> x 0 = − b ± b 2 − 4 a c 2 a (\ displaystyle x_ (0) = (\ frac (- b \ pm (\ sqrt (b ^ (2) - 4ac))) (2a))) </Dd> </Dl> <Dd> x 0 = − b ± b 2 − 4 a c 2 a (\ displaystyle x_ (0) = (\ frac (- b \ pm (\ sqrt (b ^ (2) - 4ac))) (2a))) </Dd>

Where does the minus b formula come from