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Math Analysis 1

Solved curve-sketching exercises, with graphs

Twelve complete function studies with graphs: polynomials, rational functions, exponentials, logarithms, roots, absolute values, cusps, oblique asymptotes and number of solutions of an equation with a parameter.

Curve-sketching exercises (studying the graph of a function) of the kind set in Calculus 1 (Mathematical Analysis 1) courses, in three levels: basic, with polynomials, rational functions and the exponential; intermediate, with logarithms, roots and absolute values; exam, with oblique asymptotes, cusps, split domains and discussion of the number of solutions of an equation, as in written exams. For each function the result summarises the key points; the complete solution, step by step and with the graph, opens with a click. The proofs of the criteria used are in the in-depth study linked at the top.

In the graphs the red dots are maxima and minima, the green ones inflection points, the black ones intersections with the axes; asymptotes are dashed in red.

Basic level

Exercise 1. A cubic polynomial

Study f(x) = x^3 - 3x and sketch its graph.

Result: odd; local maximum (-1, 2), local minimum (1, -2), inflection point (0, 0); no asymptotes.

Solution

1. Domain and symmetry. \mathbb R. f(-x) = -x^3 + 3x = -f(x): the function is odd, the graph is symmetric about the origin.

2. Zeros and sign. x(x^2 - 3) = 0 for x = 0 and x = \pm\sqrt 3 \approx \pm 1.73. f \gt  0 for -\sqrt 3 \lt  x \lt  0 and for x \gt  \sqrt 3.

3. Limits and asymptotes. f \to \pm\infty as x \to \pm\infty. No asymptotes: f(x)/x = x^2 - 3 \to +\infty.

4. First derivative. f'(x) = 3x^2 - 3 = 3(x - 1)(x + 1): positive for |x| \gt  1, negative for |x| \lt  1. Increasing on (-\infty, -1) and (1, +\infty), decreasing on (-1, 1). Local maximum f(-1) = 2, local minimum f(1) = -2.

5. Second derivative. f''(x) = 6x: concave for x \lt  0, convex for x \gt  0. Inflection point at the origin, with tangent of slope f'(0) = -3.

6. Graph. The maximum and minimum are only local, because the function is unbounded.

Graph of x cubed minus 3 x
Local maximum at minus 1, local minimum at 1, inflection point at the origin; the graph crosses the x axis at minus root 3, 0 and root 3.

Exercise 2. A rational function with a horizontal asymptote

Study f(x) = \dfrac{x}{x^2 + 1}.

Result: odd; asymptote y = 0; absolute minimum \left(-1, -\frac{1}{2}\right), absolute maximum \left(1, \frac{1}{2}\right); inflection points at x = 0 and x = \pm\sqrt 3.

Solution

1. Domain and symmetry. The denominator never vanishes: domain \mathbb R. Odd.

2. Zeros and sign. Zero at x = 0; sign of x.

3. Limits and asymptotes. Numerator degree lower than denominator degree: f \to 0 as x \to \pm\infty, horizontal asymptote y = 0.

4. First derivative. f'(x) = \dfrac{(x^2 + 1) - x\cdot 2x}{(x^2 + 1)^2} = \dfrac{1 - x^2}{(x^2 + 1)^2}. Positive for |x| \lt  1. Decreasing on (-\infty, -1), increasing on (-1, 1), decreasing on (1, +\infty). Minimum f(-1) = -\frac{1}{2}, maximum f(1) = \frac{1}{2}. They are absolute: the function tends to 0 at the ends and never exceeds these values.

5. Second derivative. f''(x) = \dfrac{2x(x^2 - 3)}{(x^2 + 1)^3}. The sign is that of x(x^2 - 3): negative for x \lt  -\sqrt 3, positive between -\sqrt 3 and 0, negative between 0 and \sqrt 3, positive beyond. Three inflection points: (0, 0) and \left(\pm\sqrt 3, \pm\frac{\sqrt 3}{4}\right).

6. Graph. The graph crosses the asymptote at the origin: a horizontal asymptote only describes the behaviour at infinity.

Graph of x over x squared plus 1
Minimum at minus 1 and maximum at 1, inflection points at minus root 3, 0 and root 3; the graph approaches the x axis on both sides.

Exercise 3. The Gaussian

Study f(x) = e^{-x^2}.

Result: even, always positive; asymptote y = 0; absolute maximum (0, 1); inflection points at x = \pm\frac{1}{\sqrt 2}.

Solution

1. Domain and symmetry. \mathbb R; even.

2. Sign. An exponential is always positive: no zeros.

3. Limits. -x^2 \to -\infty, so f \to 0: asymptote y = 0 on both sides.

4. First derivative. f'(x) = -2x\,e^{-x^2}: increasing for x \lt  0, decreasing for x \gt  0. Absolute maximum f(0) = 1.

5. Second derivative. f''(x) = (-2 + 4x^2)e^{-x^2} = 2(2x^2 - 1)e^{-x^2}: negative for |x| \lt  \frac{1}{\sqrt 2}, positive outside. Inflection points at x = \pm\frac{1}{\sqrt 2} \approx \pm 0.707, where f = e^{-1/2} \approx 0.607.

6. Graph. It is the bell curve of the normal distribution (up to constants): the inflection points lie one “standard deviation” from the centre, here \sigma = \frac{1}{\sqrt 2}.

Graph of e to the minus x squared
The bell has its maximum at 0 and the two inflection points at plus and minus 1 over root 2; it tends to zero on both sides.

Intermediate level

Exercise 4. Vertical and oblique asymptotes

Study f(x) = \dfrac{x^2}{x - 1}.

Result: asymptotes x = 1 and y = x + 1; local maximum (0, 0), local minimum (2, 4); concave for x \lt  1, convex for x \gt  1, no inflection point.

Solution

1. Domain. x \ne 1. No symmetry (the domain is not symmetric).

2. Zeros and sign. Double zero at x = 0. The numerator is \ge 0: the sign is that of x - 1, negative to the left of 1 and positive to the right.

3. Limits and asymptotes. x \to 1^\pm: f \to \pm\infty, vertical asymptote x = 1. Division: x^2 = (x - 1)(x + 1) + 1, so f(x) = x + 1 + \dfrac{1}{x - 1}: oblique asymptote y = x + 1, with the graph above it for x \gt  1 and below for x \lt  1.

4. First derivative. f'(x) = \dfrac{2x(x - 1) - x^2}{(x - 1)^2} = \dfrac{x(x - 2)}{(x - 1)^2}. Positive for x \lt  0 and for x \gt  2. Increasing on (-\infty, 0), decreasing on (0, 1) and on (1, 2), increasing on (2, +\infty). Local maximum f(0) = 0, local minimum f(2) = 4.

The maximum is less than the minimum: this is not a contradiction, because they lie on two branches separated by the vertical asymptote.

5. Second derivative. From the form f = x + 1 + (x - 1)^{-1}: f''(x) = \dfrac{2}{(x - 1)^3}. Concave for x \lt  1, convex for x \gt  1. No inflection point: the change of concavity happens at x = 1, which does not belong to the domain.

Graph of x squared over x minus 1
Two branches separated by the vertical asymptote x equal to 1: the left one has a maximum at the origin, the right one a minimum at 2, 4. Both approach the oblique asymptote y equal to x plus 1.

Exercise 5. Logarithm over x

Study f(x) = \dfrac{\ln x}{x} and use the result to decide which number is larger between e^\pi and \pi^e.

Result: asymptotes x = 0 and y = 0; absolute maximum \left(e, \frac{1}{e}\right); inflection point at x = e^{3/2}; e^\pi \gt  \pi^e.

Solution

1. Domain. x \gt  0.

2. Zeros and sign. Zero at x = 1; positive for x \gt  1.

3. Limits. x \to 0^+: \ln x \to -\infty and 1/x \to +\infty, the product \to -\infty: vertical asymptote x = 0. x \to +\infty: \ln x/x \to 0 (hierarchy of infinities), horizontal asymptote y = 0.

4. First derivative. f'(x) = \dfrac{\frac{1}{x}\cdot x - \ln x}{x^2} = \dfrac{1 - \ln x}{x^2}: positive for x \lt  e. Absolute maximum f(e) = \frac{1}{e} \approx 0.368.

5. Second derivative. f''(x) = \dfrac{2\ln x - 3}{x^3}: inflection point at x = e^{3/2} \approx 4.48, f = \frac{3}{2e^{3/2}} \approx 0.335; concave before, convex after.

6. Application. Since the maximum is at e, for \pi \ne e we have \frac{\ln\pi}{\pi} \lt  \frac{\ln e}{e} = \frac{1}{e}. Multiplying by e\pi \gt  0: e\ln\pi \lt  \pi, i.e. \ln(\pi^e) \lt  \ln(e^\pi), hence \pi^e \lt  e^\pi (about 22.46 against 23.14). Curve sketching also serves to prove inequalities.

Graph of log x over x
It starts from minus infinity near the y axis, crosses the x axis at 1, has its maximum at x equal to e, the inflection point at e to the three halves and then slowly decreases towards zero.

Exercise 6. A root with vertical tangents

Study f(x) = x\sqrt{4 - x^2}.

Result: domain [-2, 2], odd; absolute maximum \left(\sqrt 2, 2\right), absolute minimum \left(-\sqrt 2, -2\right); inflection point (0, 0); vertical tangents at x = \pm 2.

Solution

1. Domain and symmetry. 4 - x^2 \ge 0, i.e. -2 \le x \le 2. Odd.

2. Zeros and sign. Zeros at 0 and \pm 2; sign of x.

3. Limits and asymptotes. The domain is a closed interval and f is continuous: no asymptotes, and by the extreme value theorem it has an absolute maximum and minimum.

4. First derivative. For -2 \lt  x \lt  2:

    \[ f'(x) = \sqrt{4 - x^2} + x\cdot\frac{-x}{\sqrt{4 - x^2}} = \frac{4 - 2x^2}{\sqrt{4 - x^2}} \]

It vanishes at x = \pm\sqrt 2. Increasing on (-\sqrt 2, \sqrt 2), decreasing on (-2, -\sqrt 2) and on (\sqrt 2, 2). Maximum f(\sqrt 2) = \sqrt 2\cdot\sqrt 2 = 2, minimum -2.

At the endpoints: \lim_{x\to 2^-}f'(x) = \frac{-4}{0^+} = -\infty. By the limit-of-the-derivative theorem, at x = 2 the tangent is vertical; the same at -2.

5. Second derivative. f''(x) = \dfrac{2x(x^2 - 6)}{(4 - x^2)^{3/2}}. In the domain x^2 - 6 \lt  0, so the sign is opposite to that of x: convex for x \lt  0, concave for x \gt  0. Inflection point at the origin, with slope f'(0) = 2.

Graph of x times the square root of 4 minus x squared
The graph is defined between minus 2 and 2, where it arrives with a vertical tangent; minimum at minus root 2, maximum at root 2, inflection point at the origin.

Exercise 7. An absolute value and a corner point

Study f(x) = |x|\,e^{-x}.

Result: absolute minimum (0, 0) at a corner point; local maximum \left(1, \frac{1}{e}\right); inflection point \left(2, \frac{2}{e^2}\right); asymptote y = 0 as x \to +\infty.

Solution

1. Domain. \mathbb R. Split the absolute value: f(x) = x\,e^{-x} for x \ge 0, f(x) = -x\,e^{-x} for x \lt  0.

2. Sign. f \ge 0, zero only at 0: the origin is therefore the absolute minimum, whatever the derivative says.

3. Limits. x \to +\infty: x/e^x \to 0, asymptote y = 0. x \to -\infty: -x\,e^{-x} \to +\infty, and f(x)/x = -e^{-x} \to -\infty: no oblique asymptote.

4. First derivative. For x \gt  0: f' = (1 - x)e^{-x}. For x \lt  0: f' = (x - 1)e^{-x} \lt  0. So f decreases on (-\infty, 0), increases on (0, 1), decreases on (1, +\infty): local maximum f(1) = \frac{1}{e}.

At 0: f'(0^+) = 1, f'(0^-) = -1. Finite and different one-sided derivatives: corner point.

5. Second derivative. For x \gt  0: f'' = (x - 2)e^{-x}, inflection point at x = 2, f(2) = 2/e^2 \approx 0.271. For x \lt  0: f'' = (2 - x)e^{-x} \gt  0, convex.

Graph of the absolute value of x times e to the minus x
On the left the graph falls from plus infinity to the origin, where it forms a corner; on the right it rises to the maximum at 1, changes concavity at 2 and flattens onto the x axis.

Exam level

Exercise 8. An asymptote on one side only

Study f(x) = x\,e^{1/x}.

Result: vertical asymptote x = 0 from the right only; oblique asymptote y = x + 1; local minimum (1, e); as x \to 0^- the graph reaches the origin with a horizontal tangent; no inflection point.

Solution

1. Domain. x \ne 0.

2. Sign. Sign of x; no zeros.

3. Limits. x \to 0^+: with t = 1/x \to +\infty, f = e^t/t \to +\infty: vertical asymptote from the right. x \to 0^-: 1/x \to -\infty, e^{1/x} \to 0, and f \to 0: from the left no asymptote, the graph approaches the origin.

Oblique: m = \lim e^{1/x} = 1; q = \lim x\left(e^{1/x} - 1\right) = 1 (with t = 1/x, \frac{e^t - 1}{t} \to 1). Asymptote y = x + 1 on both sides. With Taylor, f(x) - (x + 1) = x\left(e^{1/x} - 1 - \frac{1}{x}\right) \sim x\cdot\frac{1}{2x^2} = \frac{1}{2x}: graph above the asymptote as x \to +\infty, below as x \to -\infty.

4. First derivative. f'(x) = e^{1/x} + x\,e^{1/x}\left(-\frac{1}{x^2}\right) = e^{1/x}\,\dfrac{x - 1}{x}. The sign is that of \frac{x - 1}{x}: positive for x \lt  0 and for x \gt  1. Local minimum f(1) = e.

As x \to 0^-: f'(x) = e^{1/x}\left(1 - \frac{1}{x}\right); with t = 1/x \to -\infty, e^t(1 - t) \to 0. The graph reaches the origin with a horizontal tangent.

5. Second derivative. f''(x) = \dfrac{e^{1/x}}{x^3}: concave for x \lt  0, convex for x \gt  0. No inflection point (0 is excluded).

Graph of x times e to the 1 over x
The left branch rises along the oblique asymptote y equal to x plus 1, staying below it, and arrives flat at the origin. The right branch falls from plus infinity near the y axis, has its minimum at 1, and then rises again above the oblique asymptote.

Exercise 9. A logarithm with a split domain

Study f(x) = \ln\left(x^2 - 3x + 2\right).

Result: domain x \lt  1 or x \gt  2, symmetric about the line x = \frac{3}{2}; vertical asymptotes x = 1 and x = 2; decreasing for x \lt  1, increasing for x \gt  2; concave everywhere; zeros \frac{3 \pm \sqrt 5}{2}.

Solution

1. Domain. x^2 - 3x + 2 = (x - 1)(x - 2) \gt  0: x \lt  1 or x \gt  2.

Symmetry. Substituting 3 - x for x: (3 - x)^2 - 3(3 - x) + 2 = x^2 - 3x + 2. So f(3 - x) = f(x): the graph is symmetric about the line x = \frac{3}{2}, the axis of the parabola inside the logarithm.

2. Zeros and sign. f = 0 if x^2 - 3x + 2 = 1, i.e. x = \frac{3 \pm \sqrt 5}{2} \approx 0.382 and 2.618. f \gt  0 if the argument exceeds 1: x \lt  0.382 or x \gt  2.618.

3. Limits. x \to 1^- and x \to 2^+: the argument \to 0^+, f \to -\infty: vertical asymptotes. x \to \pm\infty: f \to +\infty, but f(x)/x \to 0 (the logarithm grows more slowly than x): no oblique asymptote.

4. First derivative. f'(x) = \dfrac{2x - 3}{(x - 1)(x - 2)}. In the domain the denominator is positive: the sign is that of 2x - 3. For x \lt  1 it is negative (decreasing), for x \gt  2 positive (increasing). The zero x = 3/2 does not belong to the domain: no extrema.

5. Second derivative. f''(x) = -\dfrac{2x^2 - 6x + 5}{(x - 1)^2(x - 2)^2}. The trinomial 2x^2 - 6x + 5 has discriminant 36 - 40 \lt  0 and is always positive: f'' \lt  0, concave on both intervals.

Graph of the logarithm of x squared minus 3 x plus 2
Two branches symmetric about the line x equal to three halves: the left one falls towards the asymptote x equal to 1, the right one rises from the asymptote x equal to 2. Both cross the x axis.

Exercise 10. A cusp

Study f(x) = \sqrt[3]{x^2}\,(x - 5).

Result: cusp with local maximum at (0, 0); local minimum \left(2, -3\sqrt[3]{4}\right) \approx (2, -4.76); inflection point (-1, -6); no asymptotes.

Solution

1. Domain. \mathbb R: the cube root is defined everywhere. Write f(x) = x^{5/3} - 5x^{2/3}.

2. Zeros and sign. Zeros at 0 and 5. \sqrt[3]{x^2} \ge 0, so the sign is that of x - 5: negative for x \lt  5 (except at 0), positive after.

3. Limits. \pm\infty at the ends; f(x)/x \to +\infty: no asymptotes.

4. First derivative. For x \ne 0:

    \[ f'(x) = \frac{5}{3}x^{2/3} - \frac{10}{3}x^{-1/3} = \frac{5}{3}\cdot\frac{x - 2}{\sqrt[3]{x}} \]

Sign of \frac{x - 2}{\sqrt[3]{x}}: positive for x \lt  0 (numerator and denominator negative) and for x \gt  2; negative between 0 and 2. Local minimum f(2) = \sqrt[3]{4}\cdot(-3) \approx -4.76.

At 0: \lim_{x\to 0^+}f'(x) = \frac{-2}{0^+} = -\infty and \lim_{x\to 0^-}f'(x) = \frac{-2}{0^-} = +\infty. Infinite derivatives with opposite signs: cusp, which is also a local maximum (the function increases before and decreases after).

5. Second derivative. f''(x) = \dfrac{10}{9}\cdot\dfrac{x + 1}{x^{4/3}}. The denominator x^{4/3} = \sqrt[3]{x^4} is positive: concave for x \lt  -1, convex for x \gt  -1 (x \ne 0). Inflection point at (-1, -6).

Graph of the cube root of x squared times x minus 5
The curve rises to the origin, where it forms an upward point, falls to the minimum at x equal to 2, rises again and crosses the x axis at 5. The inflection point is at minus 1.

Exercise 11. Exponential over linear

Study f(x) = \dfrac{e^x}{x - 1}.

Result: asymptotes x = 1 and y = 0 (as x \to -\infty); local minimum (2, e^2); concave for x \lt  1, convex for x \gt  1; no inflection point.

Solution

1. Domain. x \ne 1.

2. Zeros and sign. No zeros; sign of x - 1. It meets the y axis at f(0) = -1.

3. Limits. x \to 1^\pm: f \to \pm\infty. x \to -\infty: e^x \to 0 and the denominator \to -\infty, f \to 0^-: asymptote y = 0 on the left. x \to +\infty: f \to +\infty and f(x)/x = \frac{e^x}{x(x - 1)} \to +\infty: no oblique asymptote.

4. First derivative. f'(x) = \dfrac{e^x(x - 1) - e^x}{(x - 1)^2} = \dfrac{e^x(x - 2)}{(x - 1)^2}: negative for x \lt  2 (x \ne 1), positive after. Local minimum f(2) = e^2 \approx 7.39.

5. Second derivative. f''(x) = \dfrac{e^x(x^2 - 4x + 5)}{(x - 1)^3}. x^2 - 4x + 5 has negative discriminant, so it is always positive: the sign is that of x - 1. Concave to the left of 1, convex to the right, no inflection point.

Graph of e to the x over x minus 1
On the left the graph starts from the x axis, passes through 0, minus 1 and falls towards minus infinity near x equal to 1. On the right it falls from plus infinity to the minimum at 2 and then rises quickly.

Exercise 12. How many solutions an equation has

As the real parameter k varies, how many solutions does the equation x\,e^{-x} = k have?

Result: none for k \gt  \frac{1}{e}; one for k = \frac{1}{e}; two for 0 \lt  k \lt  \frac{1}{e}; one for k \le 0.

Solution

The solutions are the abscissas of the intersections between the graph of f(x) = x\,e^{-x} and the horizontal line y = k. The graph is the one studied in the lesson:

  • f \to -\infty as x \to -\infty, f \to 0^+ as x \to +\infty;
  • increasing on (-\infty, 1), decreasing on (1, +\infty), absolute maximum f(1) = \frac{1}{e} \approx 0.368;
  • f \lt  0 for x \lt  0, f \gt  0 for x \gt  0.

Move the line from top to bottom:

1. k \gt  \frac{1}{e}: the line lies above the maximum, no solutions.

2. k = \frac{1}{e}: the line is tangent at the maximum, one solution, x = 1.

3. 0 \lt  k \lt  \frac{1}{e}: two solutions, one in (0, 1) on the increasing branch and one in (1, +\infty) on the decreasing branch (which never reaches 0). For k = 0.2 they are about 0.259 and 2.543.

4. k \le 0: the positive part of the graph no longer matters; the branch x \le 0 is increasing and goes from -\infty to 0, so there is one solution, x \le 0. For k = -0.3 it is about -0.237.

The existence of the solutions follows from the intermediate value theorem (continuous function), uniqueness on each branch from monotonicity.

Graph of x times e to the minus x with four horizontal lines
Line k1, above the maximum, does not meet the graph; k2 touches it at the maximum; k3, between zero and the maximum, meets it twice; k4, negative, only once to the left of the origin.
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