Back to homeMathematics · 2nd Bach. Science · Change subject
Start for free
BAC2 tech · Mathematics
Limits and continuity · Limits

Calculating limits

You already know how to tell when direct substitution is not enough. Now it is time to turn that catalogue of techniques into reflexes: factorising, rationalising with the conjugate, using the number $e$, replacing a trigonometric expression with its equivalent infinitesimal and, when necessary, comparing which one grows faster.

Before you start, make sure you know: Calculating limits of functions (1st Bachillerato)
Before you start, make sure you know: Ruffini's rule (1st Bachillerato)

Factorisation and the conjugate for $\frac{0}{0}$

With quotients of polynomials, the factor $(x-a)$ can always be extracted using Ruffini's rule if $x=a$ makes both numerator and denominator zero. With roots, the conjugate plays the same role.

Diagram of cancelling the common factor (x-2) between numerator and denominator to resolve a zero-over-zero indeterminate form. (x−2)·(x+1) (x−2)·(x+2) the factor (x−2) cancels: the limit is calculated with what remains
Practice★☆☆Not started
Resolve the indeterminate form $\frac{0}{0}$ by simplifying the fraction through factorising the polynomials (you can use algebraic identities or Ruffini's rule):…

Resolve the indeterminate form $\frac{0}{0}$ by simplifying the fraction through factorising the polynomials (you can use algebraic identities or Ruffini's rule):

$$\lim_{x \to 2} \frac{x^2 - 4}{x^2 - 2x}$$
Solve
Practice★★☆Not started
Calculate the following limit by resolving the indeterminate form $\infty - \infty$ using the conjugate expression: $$\lim_{x \to +\infty} \left( \sqrt{x^2 + 6x} - x…

Calculate the following limit by resolving the indeterminate form $\infty - \infty$ using the conjugate expression:

$$\lim_{x \to +\infty} \left( \sqrt{x^2 + 6x} - x \right)$$

Sketch a graph showing what this result means regarding the distance between the radical function and the line $y = x$.

You can do it on paper and send a photo with your answer.

Solve

Equivalent infinitesimals with trigonometric functions

When $x\to0$, some trigonometric functions behave exactly like a much simpler expression. Replacing them with their "equivalent infinitesimal" simplifies limits that would otherwise be very laborious.

Graphical comparison of the sine function and the identity line, tangent at the origin, to illustrate equivalent infinitesimals. sin x x near x=0, they almost overlap
Practice★★☆Not started
Calculate the limit at $x = 0$ of the following trigonometric expression, using equivalent infinitesimals or known trigonometric identities: $$\lim_{x \to 0} \frac{x…

Calculate the limit at $x = 0$ of the following trigonometric expression, using equivalent infinitesimals or known trigonometric identities:

$$\lim_{x \to 0} \frac{x \cdot \text{tg}(3x)}{\cos(x) - 1}$$
Solve
Practice★☆☆Not started
Based on comparing orders of infinity, determine the value of the following limits without performing complex operations: a) $\lim_{x \to +\infty} \frac{e^x}{x^{10}}$ b)…

Based on comparing orders of infinity, determine the value of the following limits without performing complex operations:

  1. a) $\lim_{x \to +\infty} \frac{e^x}{x^{10}}$
  1. b) $\lim_{x \to +\infty} \frac{\ln(x)}{x^2}$
  1. c) $\lim_{x \to +\infty} \frac{2^x}{3^x}$
Solve

The number $e$: the indeterminate form $1^\infty$

A base that tends to $1$ raised to an exponent that tends to infinity is not automatically equal to $1$: the result depends on the "speed" at which the base approaches $1$.

Practice★★★Not started
Study the behaviour of the function $f(x) = \left( \cos x \right)^{1/x^2}$ as $x$ approaches zero. To do so, calculate: $$\lim_{x \to 0} (\cos x)^{1/x^2}$$ Explain why…

Study the behaviour of the function $f(x) = \left( \cos x \right)^{1/x^2}$ as $x$ approaches zero. To do so, calculate:

$$\lim_{x \to 0} (\cos x)^{1/x^2}$$

Explain why this limit leads to an indeterminate form of the type $1^\infty$ and solve it using natural logarithms or the specific formula for this type of limit.

Solve
Practice★★★Not started
A multiplayer video game server has detected that the number of active users, $N(t)$, in thousands, follows an exponential growth model as a function of time $t$ (in…

A multiplayer video game server has detected that the number of active users, $N(t)$, in thousands, follows an exponential growth model as a function of time $t$ (in months) defined by the expression:

$$N(t) = \left( \frac{3t^2 + 5t + 1}{3t^2 + 2t} \right)^{2t}$$

Calculate the limit of active users as time tends to infinity ($t \to +\infty$). Does the player population stabilise at a finite value or grow indefinitely?

0 t (months) N(t) (thousands) L = ? N(t) t → +∞
Solve

Parameters and classification of discontinuities

Finding an unknown parameter in a function and classifying the type of discontinuity that remains are the two questions that come up most often, combining everything above.

Graph showing a removable discontinuity at x=a with a finite limit and the point value displaced. limit L f(a) ≠ L x = a: two different values of k give two different discontinuities
Practice★★★Not started
Let the function be $g(x) = \dfrac{x^3 - 2x^2 + k}{x^2 - 4}$. a) Calculate the value of $k$ so that the limit of the function as $x \to 2$ is finite. b) Once you have…

Let the function be $g(x) = \dfrac{x^3 - 2x^2 + k}{x^2 - 4}$.

  1. a) Calculate the value of $k$ so that the limit of the function as $x \to 2$ is finite.
  1. b) Once you have found $k$, calculate that limit.
  1. c) Classify the type of discontinuity that the function has at $x = 2$ and at $x = -2$.
Solve
Practice★★☆Not started
Given the rational function depending on a parameter $k \in \mathbb{R}$: $$f(x) = \frac{x^3 - kx^2 + 4}{x-2}$$ Find the value of $k$ so that the limit…

Given the rational function depending on a parameter $k \in \mathbb{R}$:

$$f(x) = \frac{x^3 - kx^2 + 4}{x-2}$$

Find the value of $k$ so that the limit $\lim_{x \to 2} f(x)$ is finite. Once this value is found, calculate the value of the limit.

Solve

Comparing infinities and a preview: L'Hôpital's Rule

For $\dfrac{\infty}{\infty}$ with functions (not just sequences), comparing which one dominates still works: powers, exponentials and logarithms have the same order as in sequences.

Practice★★★Not started
Calculate the value of the following limit using the comparison of infinities or L'Hôpital's Rule, analysing in detail the order of magnitude of each term: $$\lim_{x \to…

Calculate the value of the following limit using the comparison of infinities or L'Hôpital's Rule, analysing in detail the order of magnitude of each term:

$$\lim_{x \to +\infty} \frac{e^x + x^{10}}{2e^x - \ln(x^5 + 1)}$$

Which term dominates the growth of the function in the numerator and which in the denominator?

Solve
Practice★★☆Not started
Calculate the value of the following limits by comparing the orders of magnitude at infinity of the functions involved (exponentials, powers, and logarithms): a)…

Calculate the value of the following limits by comparing the orders of magnitude at infinity of the functions involved (exponentials, powers, and logarithms):

  1. a) $\lim_{x \to +\infty} \frac{\ln(x^{10} + 5)}{x}$
  1. b) $\lim_{x \to +\infty} \frac{3^x}{x^2 + 100x}$
  1. c) $\lim_{x \to +\infty} \frac{e^x + x^5}{2e^x - \sqrt{x}}$
x y x → +∞ ln(x) xⁿ aˣ ORDERS OF INFINITY ln(x) ≪ xⁿ ≪ aˣ
Solve

Summary

See all the exercises in this section