# Binomial Series Question

When using a binomial series to expand $$1/\sqrt{1-x^{2}}$$ I come up with the correct answer except that I do not add the number one to my answer. Why do I have to add one to the series, should this not arise when calculating the sum?

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Dick
Homework Helper
If x is zero the result is 1. You should definitely have a 1 in the series. What DID you do?

When expanding it I come up with the following result:

$$\sum (1*3* \ldots (2n-1)*x^{2n}) / 2^{n}n!$$

$$1 + \sum ((1*3* \ldots (2n-1)*x^{2n}) / 2^{n}n!)$$

Where does the one come from?

Dick
Yes ... they separated out the $n=0$ term, because they thought the student would not understand the product [tex]1\cdot3\cdots(-1)[/itex]