Find Asymptotes, Critical, and Inflection Points

This example describes how to analyze a simple function to find its asymptotes, maximum, minimum, and inflection point.

Define a Function

The function in this example is

f(x)=3x2+6x-1x2+x-3.

First, create the function.

syms x
num = 3*x^2 + 6*x -1;
denom = x^2 + x - 3;
f = num/denom
f = 

3x2+6x-1x2+x-3

Plot the function by using fplot. The fplot function automatically shows horizontal and vertical asymptotes.

fplot(f)

Find Asymptotes

To find the horizontal asymptote of f mathematically, take the limit of f as x approaches positive infinity.

limit(f,Inf)
ans = 3

The limit as x approaches negative infinity is also 3. This result means the line y=3 is a horizontal asymptote to f.

To find the vertical asymptotes of f, set the denominator equal to 0 and solve it.

roots = solve(denom)
roots = 

(-132-12132-12)

roots indicates that the vertical asymptotes are the lines

x=-1-132

and

x=-1+132.

Find Maximum and Minimum

You can see from the graph that f has a local maximum between the points x=2 and x=0. It also has a local minimum between x=6 and x=2. To find the x-coordinates of the maximum and minimum, first take the derivative of f.

f1 = diff(f)
f1 = 

6x+6x2+x-3-2x+13x2+6x-1x2+x-32

To simplify this expression, enter the following.

f1 = simplify(f1)
f1 = 

-3x2+16x+17x2+x-32

Next, set the derivative equal to 0 and solve for the critical points.

crit_pts = solve(f1)
crit_pts = 

(-133-83133-83)

As the graph of f shows, the function has a local minimum at

x1=-8-133

and a local maximum at

x1=-8+133.

Plot the maximum and minimum of f.

fplot(f)
hold on
plot(double(crit_pts), double(subs(f,crit_pts)),'ro')
title('Maximum and Minimum of f')
text(-4.8,5.5,'Local minimum')
text(-2,4,'Local maximum')
hold off

Find Inflection Point

To find the inflection point of f, set the second derivative equal to 0 and solve for this condition.

f2 = diff(f1);
inflec_pt = solve(f2,'MaxDegree',3);
double(inflec_pt)
ans = 3×1 complex

  -5.2635 + 0.0000i
  -1.3682 - 0.8511i
  -1.3682 + 0.8511i

In this example, only the first element is a real number, so this is the only inflection point. MATLAB® does not always return the roots to an equation in the same order.

Instead of selecting the real root by indexing into inter_pt, identify the real root by determining which roots have a zero-valued imaginary part.

idx = imag(double(inflec_pt)) == 0;
inflec_pt = inflec_pt(idx)
inflec_pt = 

-13916954-2197181/3-16954-2197181/3-83

Plot the inflection point. The extra argument [-9 6] in fplot extends the range of x values in the plot so that you can see the inflection point more clearly, as the figure shows.

fplot(f,[-9 6])
hold on
plot(double(inflec_pt), double(subs(f,inflec_pt)),'ro')
title('Inflection Point of f')
text(-7,1,'Inflection point')
hold off