Porisma

Worked solution

Sketch derivative branches around two vertical asymptotes

The diagram shows a sketch of y=f(x)y=f(x). (a) State the range of x for which f is strictly decreasing. (b) Sketch y=f′(x)y=f^{\prime}(x) on a separate diagram for all x.

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Given qualitative function with vertical asymptotes x=−2 and x=2: the negative outer-left branch falls to negative infinity; the central branch rises through the origin with a horizontal tangent; the positive outer-right branch falls towards 0.
Given qualitative function with vertical asymptotes x=−2 and x=2: the negative outer-left branch falls to negative infinity; the central branch rises through the origin with a horizontal tangent; the positive outer-right branch falls towards 0.
Given qualitative function with vertical asymptotes x=−2 and x=2: the negative outer-left branch falls to negative infinity; the central branch rises through the origin with a horizontal tangent; the positive outer-right branch falls towards 0.

[2006/IJC/Promo/7]

CJC · Lecture · Example 6

(a) Read where the original graph falls

The printed Teacher answer is x<−2x<-2 or x>2x>2. The inputs x=±2x=\pm2 are vertical asymptotes, not points of the curve; they are excluded. Between them the given curve rises, passing through the origin with a horizontal stationary inflection.

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(b) Mark the domain and derivative signs

The source Teacher derivative sketch retains vertical asymptotes x=−2x=-2 and x=2x=2. It is negative outside these inputs, positive on (−2,0)(-2,0) and (0,2)(0,2), and zero at 0. The central branch touches the x-axis at the origin as a local minimum of the derivative, rather than crossing it.

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(b) Preserve all branch-end directions

For the drawn derivative, approach −2-2 from the left gives −∞-\infty and from the right +∞+\infty; approach 2 from the left gives +∞+\infty and from the right −∞-\infty. The outer derivative branches approach y=0y=0 from below in both distant tails. These are features of the Teacher sketch, justified by the source curve’s slopes, not by applying an asymptote rule without looking at the graph.

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(b) Interpret “for all x” on the original domain

Include every branch on the domain R∖{−2,2}\mathbb R\setminus\{-2,2\}, with the two asymptote inputs excluded. The whole relation is not continued through an asymptote. No exact function formula, derivative branch height or additional intercept is supplied.

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Compare your sketch

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Teacher derivative has negative outer branches and a positive central branch touching 0 at the origin. Vertical asymptotes are x=−2 and x=2; both outer tails approach 0 from below.
Teacher derivative has negative outer branches and a positive central branch touching 0 at the origin. Vertical asymptotes are x=−2 and x=2; both outer tails approach 0 from below.
Teacher derivative has negative outer branches and a positive central branch touching 0 at the origin. Vertical asymptotes are x=−2 and x=2; both outer tails approach 0 from below.

Common mistakes

The derivative records slope, not the height of the original graph. Do not join across excluded asymptote inputs.

Solution notes

Both source copies visually compared. Student solution axes/space are blank; complete blue Teacher sketches and labels retained through qualitative mathematical reconstructions with text alternatives. No printed marks. An illustrative drawing model is not asserted as the uniquely determined original function.

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