Explanation:

The given letter series is: _ aa _ ba _ bb _ ab _ aab

To find the missing letters, we need to analyze the pattern in the series. Let's break down the given series:

_ aa _ ba _ bb _ ab _ aab

The first letter in the series is missing, but we can observe that the second letter is 'a'. So, the first letter can be 'b' or 'a'. Let's consider both possibilities:

Case 1: First letter is 'b'
In this case, the series becomes:
b aa _ ba _ bb _ ab _ aab

Now, let's focus on the third letter. We can observe that the first two letters are 'b' and 'a', respectively. So, the third letter can be 'b' or 'a'. Let's consider both possibilities:

Case 1.1: Third letter is 'b'
In this case, the series becomes:
b aa b ba _ bb _ ab _ aab

Now, let's focus on the fifth letter. We can observe that the fourth letter is 'ba'. So, the fifth letter can be 'b' or 'a'. Let's consider both possibilities:

Case 1.1.1: Fifth letter is 'b'
In this case, the series becomes:
b aa b ba b bb _ ab _ aab

Now, let's focus on the seventh letter. We can observe that the sixth letter is 'bb'. So, the seventh letter can be 'b' or 'a'. Let's consider both possibilities:

Case 1.1.1.1: Seventh letter is 'b'
In this case, the series becomes:
b aa b ba b bb b ab _ aab

Now, let's focus on the ninth letter. We can observe that the eighth letter is 'ab'. So, the ninth letter can be 'b' or 'a'. Let's consider both possibilities:

Case 1.1.1.1.1: Ninth letter is 'b'
In this case, the series becomes:
b aa b ba b bb b ab b aab

Notice that the pattern is repeating 'b' and 'a' alternatively. So, the next letter should be 'b'.

So, the missing letters in this case are 'bbaab'.

Case 1.1.1.1.2: Ninth letter is 'a'
In this case, the series becomes:
b aa b ba b bb b ab a aab

Notice that the pattern is repeating 'b' and 'a' alternatively. So, the next letter should be 'b'.

So, the missing letters in this case are 'bbaab'.

Therefore, if the first letter is 'b', the missing letters are 'bbaab'.

Case 1.1.2: Fifth letter is 'a'
In this case, the series becomes:
b aa b ba a bb _ ab _ aab

Now, let's focus on the seventh letter. We can observe that the sixth letter is 'bb'. So, the seventh letter can be 'b' or 'a'. Let's consider both possibilities:

Case 1.1.2.1: Seventh letter is 'b