1. Formulate equations using the coordinates of the given points on the graph.
Given f(x)=ax3+bx2+cx+d.
Since the graph passes through (−3,4):
f(−3)=a(−3)3+b(−3)2+c(−3)+d=4
−27a+9b−3c+d=4— (1)
Since the graph passes through (1,8):
f(1)=a(1)3+b(1)2+c(1)+d=8
a+b+c+d=8— (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=f(x)1 has a turning point at (2,41).
This means that when x=2, y=41.
f(2)1=41⟹f(2)=4
Substitute x=2 into f(x):
a(2)3+b(2)2+c(2)+d=4
8a+4b+2c+d=4— (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)]−1:
dxdy=−[f(x)]−2⋅f′(x)
For dxdy=0 at x=2, we require f′(2)=0 (since f(2)=4=0).
Differentiating f(x):
f′(x)=3ax2+2bx+c
Substitute x=2 and set to zero:
3a(2)2+2b(2)+c=0
12a+4b+c=0— (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
Solving the system (e.g. using a graphing calculator):
a=1
b=−1
c=−8
d=16
答案a=1, b=−1, c=−8, d=16