A function f\text{f} is defined by f(x)=ax3+bx2+cx+d\text{f}(x) = ax^3 + bx^2 + cx + d. The graph of y=f(x)y = \text{f}(x) passes through the points (−3,4)(-3, 4) and (1,8)(1, 8). Given that the graph of y=1f(x)y = \frac{1}{\text{f}(x)} has a turning point at (2,14)(2, \frac{1}{4}), find the values of aa, bb, cc and dd.

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Worked solution

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1. Formulate equations using the coordinates of the given points on the graph.

Given f(x)=ax3+bx2+cx+d\text{f}(x) = ax^3 + bx^2 + cx + d.
Since the graph passes through (−3,4)(-3, 4):
f(−3)=a(−3)3+b(−3)2+c(−3)+d=4\text{f}(-3) = a(-3)^3 + b(-3)^2 + c(-3) + d = 4
−27a+9b−3c+d=4— (1)-27a + 9b - 3c + d = 4 \quad \text{--- (1)}

Since the graph passes through (1,8)(1, 8):
f(1)=a(1)3+b(1)2+c(1)+d=8\text{f}(1) = a(1)^3 + b(1)^2 + c(1) + d = 8
a+b+c+d=8— (2)a + b + c + d = 8 \quad \text{--- (2)}

2. Use the position of the turning point of $y = \frac{1}{\text{f}(x)}$ to formulate a third equation.

The graph of y=1f(x)y = \frac{1}{\text{f}(x)} has a turning point at (2,14)(2, \frac{1}{4}).
This means that when x=2x = 2, y=14y = \frac{1}{4}.
1f(2)=14  ⟹  f(2)=4\frac{1}{\text{f}(2)} = \frac{1}{4} \implies \text{f}(2) = 4

Substitute x=2x = 2 into f(x)\text{f}(x):
a(2)3+b(2)2+c(2)+d=4a(2)^3 + b(2)^2 + c(2) + d = 4
8a+4b+2c+d=4— (3)8a + 4b + 2c + d = 4 \quad \text{--- (3)}

3. Use the turning point property to deduce $\text{f}'(2) = 0$ and formulate the fourth equation.

At a turning point, the derivative is zero. For y=[f(x)]−1y = [\text{f}(x)]^{-1}:
dydx=−[f(x)]−2⋅f′(x)\frac{\text{d}y}{\text{d}x} = -[\text{f}(x)]^{-2} \cdot \text{f}'(x)
For dydx=0\frac{\text{d}y}{\text{d}x} = 0 at x=2x = 2, we require f′(2)=0\text{f}'(2) = 0 (since f(2)=4≠0\text{f}(2) = 4 \neq 0).

Differentiating f(x)\text{f}(x):
f′(x)=3ax2+2bx+c\text{f}'(x) = 3ax^2 + 2bx + c

Substitute x=2x = 2 and set to zero:
3a(2)2+2b(2)+c=03a(2)^2 + 2b(2) + c = 0
12a+4b+c=0— (4)12a + 4b + c = 0 \quad \text{--- (4)}

4. Solve the system of four linear equations simultaneously.

We have the system of linear equations:
{−27a+9b−3c+d=4a+b+c+d=88a+4b+2c+d=412a+4b+c=0\begin{cases} -27a + 9b - 3c + d = 4 \\ a + b + c + d = 8 \\ 8a + 4b + 2c + d = 4 \\ 12a + 4b + c = 0 \end{cases}

Solving the system (e.g. using a graphing calculator):
a=1a = 1
b=−1b = -1
c=−8c = -8
d=16d = 16

Answer

a=1a = 1, b=−1b = -1, c=−8c = -8, d=16d = 16

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