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Find the equation of the function f(x) = -x3 + ax2 + bx + c given that it has a point of inflection at (0, 27) and f(-3) = 0.

Mathematics

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Answer

f(x)=−x3+18x+27f(x) = -x^3 + 18x + 27

Step-by-step solution

3 steps

let's determine the equation of the function f(x)f(x).

Given the function f(x)=−x3+ax2+bx+cf(x) = -x^3 + ax^2 + bx + c.

  1. Use the information about the point of inflection.

    A point of inflection occurs where the second derivative f′′(x)f''(x) is equal to zero.
    First, find the first derivative f′(x)f'(x):

    f′(x)=−3x2+2ax+bf'(x) = -3x^2 + 2ax + b

    Next, find the second derivative f′′(x)f''(x):

    f′′(x)=−6x+2af''(x) = -6x + 2a

    The point of inflection is at (0;27)(0; 27), so f′′(0)=0f''(0) = 0.

    −6(0)+2a=0-6(0) + 2a = 0 2a=02a = 0 a=0a = 0

    Also, the point (0;27)(0; 27) lies on the function f(x)f(x), so f(0)=27f(0) = 27.

    f(0)=−(0)3+a(0)2+b(0)+c=27f(0) = -(0)^3 + a(0)^2 + b(0) + c = 27 c=27c = 27

    Now we know a=0a=0 and c=27c=27. Substitute these values back into f(x)f(x):

    f(x)=−x3+(0)x2+bx+27f(x) = -x^3 + (0)x^2 + bx + 27 f(x)=−x3+bx+27f(x) = -x^3 + bx + 27
  2. Use the information f(−3)=0f(-3) = 0.

    Substitute x=−3x=-3 into the updated f(x)f(x):

    f(−3)=−(−3)3+b(−3)+27=0f(-3) = -(-3)^3 + b(-3) + 27 = 0 −(−27)−3b+27=0-(-27) - 3b + 27 = 0 27−3b+27=027 - 3b + 27 = 0 54−3b=054 - 3b = 0 3b=543b = 54 b=543b = \frac{54}{3} b=18b = 18
  3. Write the final equation of f(x)f(x).

    We have found a=0a=0, b=18b=18, and c=27c=27. Substitute these values into the original form of f(x)f(x):

    f(x)=−x3+(0)x2+(18)x+27f(x) = -x^3 + (0)x^2 + (18)x + 27 f(x)=−x3+18x+27f(x) = \boxed{-x^3 + 18x + 27}
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