Let \(f\) be differentiable with \(f(0)=0\) and \(f'(x)=(x^2-1)(x^2-4)\). The end behavior is \(f(x)\to-\infty\) as \(x\to-\infty\) and \(f(x)\to\infty\) as \(x\to\infty\).
Give a complete exact graph blueprint without finding a formula for \(f\):
a) classify all four critical numbers;
b) differentiate \(f'\) and find all inflection-point x-coordinates;
c) state every maximal monotonicity and concavity interval; and
d) list the significant x-values in left-to-right order, including the anchor \((0,0)\), so another student could sketch the graph from your blueprint.
Hints
- Use the ordered simple zeros of \(f'\) to build its sign chart.
- Differentiate \(f'\) before building the concavity chart.
- Merge the two ordered breakpoint lists only after both charts are correct.
- The anchor and stated end behavior place the qualitative graph; no integration is needed.
Solution
1. The critical numbers are \(-2,-1,1,2\). Because all four zeros of \(f'\) are simple, its sign alternates. Since \(f'>0\) for large \(|x|\), the sign pattern is \(+,-,+,-,+\).
2. Therefore \(x=-2\) and \(x=1\) are local maxima, while \(x=-1\) and \(x=2\) are local minima. The maximal increasing intervals are \(( -\infty,-2)\), \((-1,1)\), and \((2,\infty)\); the maximal decreasing intervals are \((-2,-1)\) and \((1,2)\).
3. Differentiate: \(f''(x)=4x^3-10x=2x(2x^2-5)\). Thus the inflection inputs are \(-\sqrt{5/2},0,\sqrt{5/2}\). Each zero is simple, so concavity changes at each one.
4. Testing signs gives concave down on \(( -\infty,-\sqrt{5/2})\), concave up on \((-\sqrt{5/2},0)\), concave down on \((0,\sqrt{5/2})\), and concave up on \((\sqrt{5/2},\infty)\).
5. Left-to-right significant inputs are \(-2,-\sqrt{5/2},-1,0,1,\sqrt{5/2},2\). At \(x=0\), the graph passes through \((0,0)\) and changes concavity. Combine this order with the stated end behavior and interval directions.
Answer
Local maxima at \(x=-2,1\); local minima at \(x=-1,2\). Increasing on \(( -\infty,-2)\), \((-1,1)\), \((2,\infty)\); decreasing on \((-2,-1)\), \((1,2)\). Inflection inputs \(-\sqrt{5/2},0,\sqrt{5/2}\). Concave down on \(( -\infty,-\sqrt{5/2})\) and \((0,\sqrt{5/2})\); concave up on \((-\sqrt{5/2},0)\) and \((\sqrt{5/2},\infty)\). Significant order: \(-2,-\sqrt{5/2},-1,(0,0),1,\sqrt{5/2},2\), with left end down and right end up.