Question 1179883
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Re-write your equation in the form


    f(x+1) = f(x) - 3x^2.


This equation allows you to move one step forward and calculate f(x+1), as soon as you know x and f(x).


This formula is recursive and allows you to move as far in x-axis as you want.
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So, if next time you will have similar assignment, you just know how to solve it.