# 1, 1, 2, 3, 4, 5, 8, 13, 21 - which one doesn't belong to this series?

Fibonacci Sequence is a sequence in which each term is the sum of the previous two terms. These were invented to keep the sound of pair of rabbits born each year.

## Answer: In the given sequence, 4 doesn't belong to the series.

Let's understand the solution in detail.

**Explanation:**

From the given numbers in the sequence, we can very well predict that it must be a Fibonacci series.

As we know, in Fibonacci sequences, a term is equal to the sum of the previous two terms.

If the first number is given by f_{0}, and the second number by f_{1}, then the third number is given by f_{2} = f_{0} + f_{1}.

Hence, from the given series, we see that:

⇒ 1 + 1 = 2

⇒ 1 + 2 = 3

⇒ 2 + 3 = 5

⇒ 3 + 5 = 8

⇒ 5 + 8 = 13

⇒ 8 + 13 = 21

From the above calculations, we see that 4 doesn't fit in the series.