What is x = in sequence 1.1.2.3.5.8.x.21.34.55? Write about the process, thank you!

thirteen
This is called Fibonacci sequence~
Each is the sum of the first two~
From the rabbit problem
That is to say, every rabbit can have a rabbit one month after birth
There was a rabbit in the first month
The second month was one
There will be two in the third month
The fourth month is three
And so on~

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