# combinatorics – Find the number of balanced parentheses sequences using Catalan Numbers

Need to find the number of balanced parentheses sequences in the length of 200, starts with (((.

First, I defined ‘(‘ as 0 and ‘)’ as 1.

Then I defined S to be the group of all balanced sequences, according to Catalan numbers, |S| = $$frac{1}{101}200choose100$$.

After that, I wanted to subtract all the sequences that does not fit.

The first type is those starts with 0,1,….

So we want all the balanced sequences in the lentgh of 198, that is $$frac{1}{100}198choose99$$.

Now the second type I can think of is the sequences starts with 0,0,1….. because we want to ensure that 0,0 doesn’t followed by another 0.

And there I got stuck. It is impossible to create a balanced sequence in the length of 197 so I can’t use the previous method.

Maybe I’m not approching the problem the right way?

By-the-way, the correct answer is $$frac{1}{101}200choose100$$$$frac{1}{50}198choose99$$, which I assume to be $$frac{1}{101}200choose100$$$$frac{1}{100}198choose99$$$$frac{1}{100}198choose99$$.