f(x) = 2·x^2 - 4·x
f(x) = 2·(x^2 - 2·x)
f(x) = 2·(x^2 - 2·x + 1 - 1)
f(x) = 2·(x^2 - 2·x + 1) - 2
f(x) = 2·(x - 1)^2 - 2
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f(x) = 1/6·x^2 - 2·x - 5
f(x) = 1/6·(x^2 - 12·x) - 5
f(x) = 1/6·(x^2 - 12·x + 36 - 36) - 5
f(x) = 1/6·(x^2 - 12·x + 36) - 5 - 6
f(x) = 1/6·(x - 6)^2 - 11