Calculus How To

Ratio Test: Simple Definition & Example

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What is a Ratio?
The Ratio Test, Formula and Example
Related Terms

What is a Ratio?

A ratio is a way of showing how two numbers or quantities relate to each other. For example, you can write “one dog and three cats” as 1:3 (for every dog there are 3 cats). Although using a colon(:) is the most common way to write a ratio, you can also write them as:

  • A Fraction: 1/4 are dogs and 3/4 are cats,
  • A Decimal: 0.25 are dogs and 0.75 are cats,
  • A percentage: 25% are boys (0.25 as a percentage).

If you want to figure out which number comes first in a ratio, make your problem into a “x out of y” statement. For example, let’s say you had 12 pieces of fruit, and 6 are apples. There are “6 apples out of 12 pieces of fruit”. Then just take out the excess words so it’s simply “6 out of 12.” Then you can rewrite that ratio in any form you like (e.g. 6/12 or 6:12). Just make sure to keep the numbers in the original order you had in your statement.

Another example: Out of 100 people, 76 like chocolate. This becomes “76 out of 100 people like chocolate” or 76/100.

What is the Ratio Test?

The ratio test tells you whether a series converges (i.e. settles on a certain number) or diverges (does not settle on a certain number).  The test is also known as d’Alembert’s ratio test (Jean le Rond d’Alembert is credited with first publishing the test) or as the Cauchy ratio test.

The ratio test is particularly useful for series with factorials (!). You can use the test when:

  1. Each term is a real number or complex number and
  2. an is nonzero when n is large.

The ratio test requires that you find a limit, “L”.  Once you have found L, you can then figure out whether the series converges or diverges.  The ratio test is similar to the limit comparison test but is only used when the series to be compared against equals 1:

If the ratio equals 1, then the test basically tells you nothing about convergence or divergence.  The reason that the test is inconclusive in this case is that even two series with exactly the same successive ratios can have different convergent properties when the limit of the successive ratios are equal to 1.  A limit of 1 shows that the terms do not behave as a geometric series in the limit. If you do get a series with a ratio equal to 1, then choose another test for convergence.


The ratio test is represented algebraically as:
ratio test

The formula above tells you to:

  1. Form a ratio of an + 1/an,
  2. Take the absolute value of the ratio and the limit as n → ∞.
  3. Compare L to 1 and determine whether the series converges or diverges.

Example: Using the Ratio Test

Example question: Does the following sequence converge or diverge?

This sequence has a summation symbol(Σ) and a factorial (!). If you aren’t sure what those are, you may want to read about summation notation and factorials first.

Step 1: Form a ratio, following the formula.
ratio test

  • The denominator (an) is the original sequence (the one given in the question).
  • For the numerator (an + 1), you’re adding a “+1” everywhere you see an n.
  • Don’t forget to add the absolute value symbol.

If you aren’t sure how to do this, the following graphic shows you the steps:

Step 2: Find the limit of Step 1 as n goes to infinity (n → ∞). This is where you might have difficulty, as it does require some strong algebra skills (no calculus needed at this point!). Here are the basic algebraic steps:

As for the limit, notice that we’re looking for the limit as n goes to infinity. As n gets larger, the denominator is going to get larger. That means it’s going towards 2/∞, which is essentially zero. Therefore, the limit is zero.

Step 3: Compare your answer in Step 2 to 1 and determine whether the series converges or diverges. As the limit is 0, the ratio test rules say that if the ratio is less than 1, the series converges absolutely.

Related Terms

  • Rate ratio (epidemiology): compares the incidence of events happening at different times.
  • Ratio Scale: a scale of measurement where the difference between values is meaningful, and zero is also meaningful.
Stephanie Glen. "Ratio Test: Simple Definition & Example" From Calculus for the rest of us!

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