11: You should get

In order to use the exponential expansion, two problems need to be addressed. Get rid of the "+1" in the exponent and fix the beginning of indexing using the add-subtract trick.

The alternative solution is based on the observation that if there were just k in the numerator, then it would cancel with the factorial in the denominator. However, this can be achieved easily by splitting the fraction into two. Then it is just a matter of adjusting indexing in the first resulting series and the rest should be easy.

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