Tuesday, February 9, 2016

Give an example of a function, f(x), that has an inflection point at (1,4)

The zeroes of the 2nd derivative represents the inflection
points of a function.


The order of the function to be
differentiated is 3rd order.


Let f(x) = ax^3 + bx^2 + cx +
d


We'll differentiate f(x) with respect to
x:


f'(x) = 3ax^2 + 2bx +
c


We'll differentiate with respect to x
again:


f"(x) = 6ax + 2b


We'll
cancel f"(x) = 0 knowing that the zero of f"(x) is x =
1.


6a + 2b = 0


3a + b =
0


b = -3a


We'll compute f'(1)
= 3a + 2b + c => f'(1) = b + c


We'll compute f(1) =
a + b + c + d


We also know that f(1) = 4 => a + b +
c + d = 4


Since the number of unknown coefficients is
larger than the number of possible equations, the function cannot be determined under
the circumstances.


Therefore, any polynomial
of 3rd order, at least: f(x) = ax^3 + bx^2 + cx + d, could have an inflection point
(1,4), under given circumstances.

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