# n-grams textbook question

I have this question I found regarding n-gram modeling in the Speech and Language Processing textbook by Daniel Jurafsky:

Suppose we didn’t use the end-symbol </s>. Train an unsmoothed bigram grammar on the following training corpus without using the end-symbol </s>:
<s> a b
<s> b b
<s> b a
<s> a a
Demonstrate that your bigram model does not assign a single probability distribution across all sentence lengths by showing that the sum of the probability of the four possible 2-word sentences over the alphabet {a,b} is 1.0, and the sum of the probability of all possible 3 word sentences over the alphabet {a,b} is also 1.0.

Please can you verify that is my approach correct?

• @D.W.♦ Please can you help? – Ohanzee May 13 '20 at 13:58
• I'm not sure I understand how to interpret $P\{x_1|x_2, x_3\}$, especially when I think you are assuming a probability of $0/1$ = 0 (what makes you do so?). – greybeard May 13 '20 at 14:08
• I don't see an approach. We don't grade answers or check whether they're correct; that's not the purpose of this site. Also, we discourage "here is my exercise problem, how do I solve it?" style questions. – D.W. May 13 '20 at 21:38