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The graph of \(f\) is shown with tangent lines \(t_1\) at \(A(0,2)\) and \(t_2\) at \(B(4,2)\).
a) Use the tangent lines to find \(f'(0)\) and \(f'(4)\). Briefly explain your method.
b) Decide whether the derivative is positive, negative, or zero at each input. Briefly justify each answer.
1) \(x=2\)
2) \(x=5\)
3) \(x=-2\)
Hints
- A derivative value equals the slope of the tangent line.
- Use two exact points on each tangent to calculate its slope.
- Increasing, decreasing, and horizontal behavior correspond to positive, negative, and zero derivative values.
Solution
1. The line \(t_1\) passes through \(A(0,2)\) and \((-2,0)\). Its slope is \(\frac{2-0}{0-(-2)}=1\), so \(f'(0)=1\).
2. The line \(t_2\) passes through \(B(4,2)\) and \((6,0)\). Its slope is \(\frac{0-2}{6-4}=-1\), so \(f'(4)=-1\).
3. At \(x=2\), the graph has a local maximum and a horizontal tangent, so \(f'(2)=0\).
4. At \(x=5\), the graph is decreasing, so \(f'(5)<0\).
5. At \(x=-2\), the graph is increasing, so \(f'(-2)>0\).
Answer
a) \(f'(0)=1\) and \(f'(4)=-1\)
b)
1) \(f'(2)=0\)
2) \(f'(5)<0\)
3) \(f'(-2)>0\)
