Sketch a function from its derivatives 1
Instead of a freehand sketch, give an exact **graph blueprint**.
A differentiable function satisfies \(f(0)=1\), \(f'(x)<0\) for \(x<0\), \(f'(0)=0\), \(f'(x)>0\) for \(x>0\), and \(f''(x)>0\) for every real \(x\).
Describe the graph from left to right. Your blueprint must state the monotonicity and concavity on each side of \(x=0\), and identify the type and coordinates of the point at \(x=0\).
Hints
- The sign of \(f'\) gives the direction of travel.
- A negative-to-positive change in \(f'\) gives a local minimum.
- The sign of \(f''\) gives concavity.
- Use \(f(0)=1\) to place the landmark vertically.
Solution
1. Since \(f'(x)<0\) for \(x<0\), the graph decreases as it approaches \(x=0\) from the left.
2. Since \(f'(x)>0\) for \(x>0\), the graph increases to the right of \(x=0\). Thus \(f'\) changes from negative to positive, so \(x=0\) is a local minimum.
3. The anchor \(f(0)=1\) makes the minimum point \((0,1)\).
4. Because \(f''(x)>0\) everywhere, both the left and right branches are concave up.
Blueprint: decreasing and concave up to \((0,1)\), then increasing and concave up.
Answer
Decreasing and concave up for \(x<0\); local minimum at \((0,1)\); increasing and concave up for \(x>0\).
