f(x) = ax^2 + bx + c
f(x) = a(x^2 + b/a*x) + c
f(x) = a(x^2 + b/a*x + (b/(2a))^2 - (b/(2a))^2) + c
f(x) = a(x^2 + b/a*x + (b/(2a))^2 - b^2/(4a^2)) + c
f(x) = a(x^2 + b/a*x + (b/(2a))^2) + c - b^2/(4a)
f(x) = a(x + b/(2a))^2 + c - b^2/(4a)
Scheitelpunkt ist also bei S(-b/(2a) | c - b^2/(4a))